One of the assumptions I use for the future after peak oil is that it's static. After the recovery period, technology won't change that much and international politics will sort itself out into a highly stable configuration. This is of course absurd, one thing the human world isn't is static, not even traditional cultures are, they just seem static as that is the source of their legitimacy (mentioned here). Some of the more interesting scenarios below are caused by changes in the human world, whether that is from overshoot , technological change, various disasters, empire building or infrastructure build up. I also assume that certain things will stay much the same as they are now; no one will figure out how to quickly and easily neutralize all types of sea mines, guns will be the primary weapon as long as basic metalworking exists and missiles will still be more expensive than shells. These assumptions are more reasonable, but aren't necessarily true. And so on.
Now, some of the scenarios below are under most assumptions absurd or are straight out of sci-fi. But since we can change assumptions for mental models, I'll go over them anyway. This is just to look at what the far future may hold, and maybe guess at what the mid future could look like.
First up, what happens if a superpower of the far future (where nukes are unlikely to exist) initiates Project Thor and actually uses it to threaten everyone. Nukes may not exist because there's no accessible uranium reserves, nukes are too technologically complex or something else. Lets go over what Project Thor (also known as Rods from God) is first. The idea (from Jerry Pournelle while he was working for Boeing) is to place a satellite in orbit which can unleash tungsten (melting point is 3683.15 K) rods of varying sizes in such a way that they use earth's gravity to accelerate them onto the target. Since their going at about 9km/s they pack at lot of punch, so they're able to destroy any single structure and underground bunker the controllers want to destroy (even city blocks). Note that since it launches rods, for various aerodynamic reasons, its not exactly an area of effect weapon like a nuke is, more for small strategic strikes, though launching a swarm of small ones could cripple an army or city, doing so would likely use up all or most of an empires arsenal.
Now, this scenario is taking place in a world without fossil fuels so the question of time and expense are quite important. The project could take decades (30-60 years) to get one weapons platform in orbit (it won't be a big platform and would need replacement every 50 or so years), since fuel would have to be either created from electricity or biomass (both scarce and valuable resources) and the entire infrastructure would take a significant amount of R&D. Once the first platform is up, since most of the work is in infrastructure and capacity building, a new platform could be made every 10 years or so (meaning that their would be at most around 5 such platforms in orbit). This also means that there won't be that many in orbit and using one would be a decision only taken when absolutely necessary (like nukes today). In fact they could follow a similar pattern to how nukes have been used, the first use is to demonstrate their power and frighten everyone and after that it's only kept operational for political reason. The difference being that this time it's because of the expense of replacing them rather than both sides having them. Also since they can't easily destroy a city or hit dispersed targets, they won't have much impact on barbarians and nomadic people, mind you the conventional forces of a power capable of creating project Thor are more than enough to handle those. And for any small-medium scale disputes/wars it won't be an effective deterrence because the expense is to much for it to be worth it, however it does mean the superpower has a clear line which no other hostile power will cross and certain behavioral patterns could overcome the operational limits of project Thor. Of course since there's a cyclical nature inbuilt into the system, you want to use a platform before it's breaks down but not straight after launching, the empires opponents are likely to take advantage of that cycle and behavioral pattern it produces.
The thing is, project Thor isn't the sort of thing that could easily destroy cities (sections yes) or anything that's spread out (like an army or navy), but single important targets can be destroyed quite easily (like airbases). Other orbital weapon systems are better suited to that task, interestingly enough those weapons systems are offshoots of civilian systems. The first one is an old idea of placing rail-guns on the moon and shooting chunks of metal (or whatever goods are traded) to an extraction point before being taken to earth, in Heinlein's "The Moon is a Harsh Mistress" the loonies (lunar people) instead target earth in their revolution. The other is a natural outgrowth of asteroid mining, see Planetary resources, where you simply place thrusters on an asteroid and boost it in an intercept course for some target, note this is also a problem if basic slip ups occur (or intentional action). A power that can place project Thor in orbit, can probably also build a few of these and use them, after-all they provide basic economic benefits anyway. When used, these weapons will do the minimal amount of damage necessary to fulfill the mission for the simple reason that doing more requires significant resources, so at most a city would be flattened or a tsunami caused.
Space treaties and other such considerations (there are very valid reason to not militarize space) is looked at in a later post, but there are some key points to think of. Currently WMD are only for chemical, biological, radiological and nuclear and the above systems are kinetic. Two of them are also civilian systems that can be easily weaponized, but by their nature they can be easily weaponized. And the above systems are for attacking earth, actual space warfare isn't mentioned.
So what are the likely consequences of a future superpower deploying and at least once using project Thor. Like today, it means the superpower isn't going to fight a major war with other superpowers, since it can just threaten them with it but cannot fully coerce them, but guerrilla warfare is likely to spring up since it's the only option "Guerrilla war, not push button war, was the wave of the future - it could only flourish under the protection of the bomb" (Colonel Hackworth, About face). The superpower is likely to get a fair bit of tribute from foreign powers (protection money), which has all the usual consequences, just without having to station troops everywhere. Imagine if America was the only power in the world who had nuclear weapons, something similar could result. Mind you there would be an immense level of pressure on the other powers to develop a countermeasure, either their own project Thor's or ways to destroy the satellites or creating their own cheaper WMD's such a chemical or biological weapons. But it would rarely be actually used, in fact the superpowers main policy would be to prevent needing to use it (say by propaganda) and coasting on the fear and prestige having it grants them because its such an expensive project (in other words it would not be significantly different from modern day nukes). Also, it's a satellite so there not exactly hard to spot.
So that's one possible situation for the future, not a particularly likely one but an interesting one.
Sea mines are the most cost-effective weapons ever developed and they possess some capabilities that are sometimes ignored, such as the ability to completely shut down any shipping route (this is one of the likely consequence of an attack on Iran, and one that could knock out roughly 1/5th of the world's oil production rapidly). And they're not especially technologically complex or expensive, some of the advanced modern ones are but most aren't, so being able to build and deploy them is something the far future could easily do. Since it's unlikely that any future superpower will have the preponderance of force and indirect control that America currently possesses, important shipping lanes like the Suez canal or Malacca strait could be shut down and remain that way by the relevant powers with ease. This also applies to ports and harbors, lay a minefield down and no one is getting in or out, except if you have mine free channels (which is where you aim the artillery) or sufficient military power to clear the mines. Just to be clear, sea mines tend not to have as many civilian casualties as land mines, since everyone knows they're there (also they only hit ships) and it's easier to clean them up once the wars finished along with a greater economic incentive to do so compared to land mines and un-exploded ordinance. The last point is fairly important, in WW2 the Germans developed a new mine type that couldn't be swept (at that point in time), Hitler actually forbade it's use because he feared it would permanently block any passage it was used on.
This could easily mean that for offensive purposes, it's almost impossible to attack a prepared position from the sea. So attacking and storming ports is only likely to work if its a surprise attack, otherwise any assault ship that tries to land will explode. Landing at various beaches and creating an outpost will remain the main way to invade a country, the main difference is that this technology is going to be spread throughout the world (think about the viability of global empires now). There are some constraints though, such as shutting down all shipping in the area you place them (all commercial, fishing etc, all) which hurts you economically and the difficulty of mining an entire coastline along with wide or deep water shipping routes ( this is sufficiently demanding that not all powers could do this, instead concentrating on blocking ports and key routes). Offensively, mines are fairly useful because they shut down all shipping, Operation starvation was an attempt to starve Japan and the only reason it didn't end the war was because it was initiated too late. In the future, it's unlikely that many countries will require shipping to survive (not counting especially coastal ones such as Indonesia), so this isn't a huge threat but it could easily destroy trading powers and the projection ability of naval powers. There are countermeasures, but they cost time, effort, lives (mine-clearing isn't the safest occupation) and boats.
Airships have the potential to have some interesting effects. Firstly, they offer a great way to mitigate some of the logistical problems peak oil will cause because they can lift a fair amount of cargo efficiently and don't require extensive infrastructure. Otherwise their role will depend on missiles and aircraft. If missiles are common, then it's not going to be too hard to destroy airships, or at least ground them and so they'll mostly have roles in recon and airlift. Since missiles are fairly expensive, especially the ones that could reliable take out an airship and require electronics to be effective, its unlikely for low tech armies and non-state actors (or equivalents) to have more than a handful. So for example law enforcement (against bandits or external raiders) and border patrols could be carried out by airships fairly well even if missiles are in play. In remote areas (even in mountainous terrain), airships could replace horses as the main form of transport or even governmental office. Also, just to be clear, chances are pure airships won't be used (e.g the Hindenburg) but instead hybrid lighter-than-air(LTA)/heavier-than-air(HTA) ones since that pathway offers quite a few advantages (minimal landing infrastructure, can fly without lifting gas, etc). To understand how the hybrid works, pure LTA lift solely by buoyancy, while in pure HTA lift is generated solely by aerodynamic forces, hybrids work by both methods.
There is a resurgence of airships happening at the moment, it actually started earlier but failed due to various impracticalities. The first set of companies, like cargolifter, and pentagon projects tried to go straight to building big heavy (straight to 1000 tons of cargo capacity) airships without any smaller models, prototyping or experience. This is a problem because airship technology had basically atrophied after WW2 and it's only now catching up, building big isn't really an option until the technology is updated, a sizable workforce built and enough experience earned. Now the private market is starting to do it the proper way, solarship is building small airships and may later start building the big ones. Established companies with experience (very important that) are also starting to get involved, like Lockheed Martin. Note the established companies are building bigger but more conventional airships, solarship is building one that is powered by solar cells which necessitates smaller models to experiment with first. And most of the work is in hybrid airships, pure airships will be used but in a more niche role. Also, airships are always cheaper than comparable aircraft, it's the costs associated with airships that were expensive (mooring masts, groundcrews) which hybrids do away with.
If missiles are not in play, due to cost, technical complexity or lack of targeting systems, then other possibilities exist. Of course rockets (their not technological complex) are likely to exist and launching a swarm of them at a sufficiently exposed (close to the ground, unaware etc) airship could be a common way of destroying them, along with flak guns. Interestingly enough, the fragility of airships is their greatest defense, bullets and missiles simply pass through them without doing damage and since the gas pressure generally isn't that high, very little leakage happens and they tend to have very little in regards to radar and heat signatures, so destroying them isn't as easy as it would seem unless they try to stand their ground (however they would often be mortally damaged and so have to either go back for repairs or slowly sink into the ground over a day). Also setting them on fire (they'll use hydrogen to lift) is hard to do, WW1 fighters had to expend 1-2 drums of incendiary ammo on the same spot make the hydrogen ignite, because combustion requires oxygen as well. They could be used to supply positions deep within enemy territory, after all the only infrastructure they require is for landing/cargo transfer, and they can ignore geographical obstacles, the main way to stop them in this context would be opposing aircraft. The ability to easily cross geographic obstacles is potentially the most game changing attribute of airships, suddenly mountain ranges might not be good border lines anymore. Its unlikely to completely change borders, since it takes more than supplies to control an area (it does helps), but things will likely shift.
Using airships in a more aggressive role will be a bit harder, since the techniques that developed in WWII to fight aircraft will work just as well against airships. The effectiveness of aircraft against ships decreased during WWII because everyone eventually figured out how to defend against them, such as using a proximity fuses (radio based) to turn a 3D targeting problem into a 2D one. So chances are they aren't going to sink military warships that often, but they could aid fleets by providing reconnaissance and some fire support, supply ships or act as privateers. The main problem is probably going to be weight, weapons aren't exactly light and carrying all the ammo necessary for an extended fight could easily use up all the lifting capacity, one shot weapons like rockets will likely be common for that reason alone. So making them a dedicated weapons platform might not be the best use of resources, especially since they aren't easily armoured (unless certain biomaterials become common). They could be used in surprise attacks (one shot weapons), for harassing and taking out hard to reach but vulnerable targets. Laying mines however, is an area that they will almost certainly be used in, just fly over the area and drop mines down. Also, they excel in protecting convoys from submarines.
Lets look at mobility and the various land vehicles (also not looking at river transport here) that could exist in the future. How their used is going to be fairly important, but what about when they meet, how does that work out. Or if a military has all of them at once, integrating them could be a logistics headache, though there will likely be tricks and rules of thumb to help.
The oldest are the various pack animals; horses, donkeys, camels, mules, alpacas, oxen etc, most of which carry stuff, on their back or on a cart/wagon, but some can also be ridden in combat (camels and horses). After that you have bikes, while useful their probably not going to replace pack animals since they require a path (doesn't have to be good). Velomobiles were invented around the same time as the car, on the cargo side they could possibly replace pack animals and offer a potential weapons platform (better than a bike). Motorized (ICE, electric hybrid etc) vehicles will still be around, there simply won't be that many of them, and they will fulfill all the roles they currently do. Trains also count in this area, but since they need tracks to function they operate differently than the others. Trains are normally only used for combat in large open areas (vast expanses of Russia for example) and even then rarely in anything more than defending themselves, defending tracks from attack or as a platform for artillery, artillery can be laid on tracks as well.
So at the most basic level, how could this various vehicles be used (outside of logistics). Horse and other mounts are unlikely to ever be used for direct combat, not unless guns regress to around 16th equivalents (highly unlikely), but they'll have roles as scouts (as they are currently still used occasional in eastern Europe), dragoons and infiltrating or striking behind enemy lines. The fact is that mounts are far too vulnerable to guns or artillery to be effective, they'll only be used where that's not an issue. Bikes and velomobiles offer a much more useful platform for direct combat, and when the terrain is right for rapid transport as well, whether its towing guns or using mounted weapons. The main problem is armour, it's generally heavy and human muscle has strict limits on what's achievable, so a support role is much more likely. Mind you, some of the new materials appearing could change that, nacre is stronger than any other ceramic and is fairly light. Motorbikes are likely to remain as transport for VIPS and vital communications (hard to intercept), they cost fuel, aren't heavily armoured but are more so compared to human or animal power.
So that leaves trains and motor vehicles. Since fuel/electricity is going to rare and expensive, we can look at them from that point of view. Trains use barely any fuel, but lack flexibility and suffer problems if tracks are sabotaged/destroyed, though the tracks can be quickly replaced and its difficult to destroy enough to more than delay the trains. Trains and other tracked vehicles that aren't likely to be attacked will be electric, but they'll provide vital transport services. Motor vehicles on the other hand have flexibility and don't have dependencies on tracks, but use a lot more fuel than trains (especially heavy combat vehicles), because batteries are heavy chances are the only fully electric vehicles will be in transport or for use in bases. A note on fuel synthesized with electricity (air fuel synthesis, AFS), since electricity is easier to produce with renewables than fuel, in the future most transport will be either electric, animal or human powered but when it's necessary to use fuel (space, military vehicles, most planes etc) it would make sense to use AFS (or similar technologies) alongside biofuels.
Motor vehicles aren't going to do any heavy lifting (that includes moving themselves) when there are trains around to do it instead. But trains aren't likely to engage in direct combat either and require specific infrastructure, so they complement each other. Trains will likely be the main way of moving equipment around and will occasionally double as heavy artillery platforms, they could be used in defensive formations as mobile gun platforms (lots of machine guns and cannons) but only in well designed fortifications. Vehicles will be used for transport of heavy equipment only at the end of the journey, from the train station to it's emplacement. However they will (when applicable) see direct combat more often, their a lot easier to armour than pedal machines and don't have immediate dependencies on infrastructure (for as long as they have fuel). Chances are there won't be that many of them, and they certainly won't be used to move soldiers strategically (tactically, sometimes) but they offer advantages when used. One likely consequence of a reduced role of vehicles is a return of trench warfare (WWI) since it becomes a lot harder to break trenches without vehicles and gunpowder weapons are very good on the tactical defensive.
So what happens when armies based on different transport technologies meet?
What happens if I'm wrong and somewhere a horse based army develops because the local guns aren't effective enough to stop them. And what happens if they invade a region which has trains or is suitable for pedal machines, how would they fair (not well is the short answer). More likely is an equivalent interplay to the one that existed between the border of Parthia and Rome, one side was suited to infantry and the other to cavalry, some areas just aren't going to be suitable for pedal machines or trains and despite the numerous advantages they offer, they won't be used because of that. Of course their is the issue of fuel sources, since it can be transported fairly easily the actual geographic area the war is taking place may not matter as much as the enviroment & economy of the respective societies.
Another factor that needs to be looked at is the lack of nuclear weapons (see the project Thor section). After-all, most of the good uranium will have been mined and the vast techno-structure to build them has no economical backing what so-ever, at least going to space (Project Thor) has economical benefits like good communication, weather forecasting and extensive R&D benefits (for every $1 spent on space, $10 dollars has been earned at a minimum). So that knocks out 2 of the WMD categories (nuclear and radiological) while leaving chemical and biological and the possible addition of kinetic. Biological WMD's suffer quite a few problems outside of using natural diseases, they're hard to weaponize (you have to hit the sweet spot between incubation period and lethality and without lots of air-travel their spread would be easily curtailed) and then there's the issue of control, so we'll assume they aren't used here. Also the sweet spot changes as the relevant conditions, hygiene, demographics, other diseases etc changes, natural selection can't get it right for this reason and it's one the best optimization tools around. Chemical weapons could exist in the future, the challenge is synthesizing the vast stockpiles required and it's likely to be around WWII levels, so all the major powers have stockpiles (I'm not a chemical engineer, so this is more an educated guess), in which case we can disregard them because they didn't change the big picture that much, especially not compared to nukes.
Now to give you an idea how this could change geopolitics from what we have now, lets do a little thought experiment. What if nukes hadn't been invented (for whatever reason) and are never invented. Now the first thing would be that Hiroshima and Nagasaki wouldn't have been nuked, the US would have still won but they'd have 2 choices. Invade Japan or wait for operation starvation to force a Japanese surrender, given the 1/2 to 1 million estimated US casualties from an invasion (given the circumstances millions of Japanese would also die), chances are they'd choose to wait it out (and thus kill millions of Japanese through starvation). Or they'd surrender anyway in the same time frame because the main impetus to surrender was the Soviet union invading, see here. Either way, the end result would be the same, more casualties during WWII, Japan occupied by America, the Marshall plan initiated and 2 superpowers locked in conflict, additionally their was massed incendiary bombing on Japan that was killing far more people than the nukes (one of the major reasons Nagasaki and Hiroshima were chosen were that they were amongst the few cities still standing), one of the major results would be that Japan would have suffered far more damage. Only this time there's no reason for it to remain a cold war.
After-all, the main effects of nukes is that they make war between nuclear powers a mutually losing proposition (through MAD) and they drastically limit how intense a war can get. Yes, the US and Russia have been involved in several wars since WWII, but compared to the world wars (or even standard ones) those have been fairly tame wars. However nuclear powers seem to engage in more wars than non-nuclear powers, this is likely due to two things 1. nuclear powers tend to be either the most powerful nations or those in very precarious security situations (Israel) and 2. nukes arguably cause the stability-instability paradox whereby they increase the number of small wars that nuclear states engage in while precluding major conflicts. The civilian casualties from the Vietnam war were around 1/2 a million over a 19 year period, while the death toll from the rape of Nanking is estimated from 13,000 (Japanese guess) to 300,000 (Chinese government) and that event took place in only 6 weeks, Vietnam wasn't a brutal war despite the mythology that has spread about it (note I'm not saying that there weren't brutalities or atrocities, just that as wars go it wasn't a particularly bad one). The Iraq and Afghanistan (about 55,000 dead) wars are far closer to policing actions than actual wars, the brutal conflicts are now in Africa and aren't fought by states, see here. Nuclear weapons are some of the most successful instruments of peace and weapons ever built, even though they've only been used twice. Military power is brittle, you want to use it as little as possible and preserve it and nuclear weapons never have to be used to function. A lot of people mistake the uses of quite a few military gadgets, aircraft carriers are the same today, they aren't meant to be physically used, their success is in their ability to stop wars and influence states by their mere presence, though this depends on their ability to actually fight wars and being designed to be used so.
On the other side of the world and a few years later, America decides to re-arm Japan and back its re-invasion (with troops) of China to oust Mao Zedong and destroy communist China, to weaken the Soviets. How does it go, well China is better prepared this time round (not fighting a civil war helps) and will get lots of Soviet help, remember the Soviets controlled Manchuria in to the north (in fact the bomb itself was not enough to get the Japanese to surrender, it also required the threat of soviet invasion. On the other hand, this time Japan can focus exclusively on China and has the massive industrial might of America behind it, which includes far better technology. Massive civilian casualties will be inflicted on China no matter what, similar to in WWII, and it's unlikely that it would become the industrial power it is nowadays. Without nukes, the above scenario is easily what history could have been, empires tend to fight each other and there isn't anything holding them back. Admittedly, the Industrial economy provides some mitigation against wars, but the combatants of the World wars were by and large Industrial civilizations. How the wars turns out doesn't particularly matter for this discussion, the fact that it would likely happen and the consequences are the important bit.
My Twins version of WW3. Europe becomes for the third time in half a century 'bathed in blood' with massive tank and infantry waves crashing from the east and with Germany broken in twain and therefore unable to even attempt to check the Russian advance, most likely meaning Russia would conquer continental Europe within several months of hard bloody fighting, once again massive bombing campaigns would begin against Britain and the industrial areas of Europe. Likely then the conflict would spread out to North Africa and the Middle East, no matter the outcome millions would likely die from the fighting and tens of millions more from the immediate results.
In Asia Japan is dragged back into war as the US forces the Japanese into the war, naval clashes begin and soon after amphibious landings begin against Manchuria and another Russo-Japanese war begins, however this time the Russians aren't a pushover and it includes bombing campaigns against Japan with the Russians copying the incendiary bombing campaign of the Americans during WW2. Later into the war after the CCP gains control over China (5+ months into the conflict) it decides to take advantage of the situation to create a network of client states in Asia, starting by beginning the Korean war a year early, quickly conquering South Korea and handing it over to the North Korean government then moving onto similarly securing the allegiance of Laos, Vietnam, Burma and the various small neighboring states removing any traces of colonial rule and installing communist dictators into them before finally moving in and conquering Tibet.
The likely final result of such a conflict would be the USSR expanding to include continental Europe, North Africa and parts of the Middle East (and unfortunately for the occupied people's Stalin is ruthless enough to stop any uprisings or guerrilla wars after the conclusion of the war), China having its own little communist empire in South East Asia and America only having hegemony across the entire American continent, the Pacific and the remnants of the colonial empires empire including Britain, Australia and the bottom part of South East Asia. the cost of such a conflict would be tens of millions dead immediately and tens if not hundreds more from the attendant affects and repression from the various communists governments.
This situation leads to a very uneasy peace punctuated by regular violent clashes between the major powers, such as the violent three way decade long war over India, with the added resources from its conquests the USSR is able to hang on for an even longer time leading to it remaining into the early 21st century, China follows a remarkably similar trajectory in this future with increasing antagonism towards the USSR and with the death of Mao Zedong and the rule of Deng Xiaoping China begins to open up its economy and become very similar to its current socialist-capitalist hybrid that it is now. Swiftly these reforms are copied across East Asia leading to rapid economic growth for China and its client states. The situation is not so good for Europe as the USSR is unable and unwilling to enact a Marshall plan leading to Europe becoming permanently stuck into a semi-developed status.
However with the relative peace that descends all the major powers begin to prosper economically with oil production following a similar trajectory in this universe as ours, with two key differences, firstly oil is a non-fungible commodity, the majority of it is produced within the three power's spheres of influence and is reserved for them, secondly there are no 1970's oil crises as the entirety of the middle east is under some form of imperial jurisdiction able and willing to violently ensure the continuation of oil production. This means that peak oil arrives sooner with the USSR feeling its impacts first as Russian oil production peaks in the late 1980's and continues a slow ragged decline from their, causing the USSR to slow in its economic growth, causing massive internal tensions and conflicts, though noticeably not enough to cause it to break up and it is still able to quash any attempts at succession, the US goes through a similar though massively less strenuous and violent process with its peak production in 1970. Soon after in the early 2000's the Middle East reaches its peak production causing problems for all three of the world powers as the USSR and US loss direct access to oil production and thus China and its clients face fiercer competition for the few oil resources produced by 'independent' states. By the 2010's the situation between the powers is deteriorating, further fueled by the 'wild card' affects of climate change, and from the looks of it all out war between the major powers over the remaining oil reserves seems inevitable and imminent.
So what does this tell us about future societies without access to nukes (and it should be noted that at best only a few states would have nukes anyway); well the major powers will likely fight far more wars that are far more vicious than they do now and minor powers will continue to fight wars amongst themselves and against the major powers. This is the actual reality of the future, not the one posited by such Utopian thinkers as Dimitry Orlov, who in his book The Five Stages of Collapse posits the end of warfare based on the current spread of easily accessible high tech weapons (never mind that most groups with access to them receive massive state based support) and the ability of asymmetric forces to "endlessly frustrating" conventional forces, something that is only true due to our morality and the unwillingness of the US to fight some of its wars properly (but for understandable reasons).
That Aircraft carriers and nukes are "irrelevant" (never mind that most recent wars have not seen their use and so their relevance is itself irrelevant) and that mechanized infantry are becoming harder to operate due to rising fuel costs (never-mind that non-mechanized infantry have been used in industrial conflicts and will be able to operate in the post-peak world). As well the "spread and increasing effectiveness of war in cyberspace" (The problem with this argument is that cyber warfare has very hard limits on its capabilities and is irrelevant to the post-peak future due to the likely demise of the internet). He additionally has the mistaken belief that the purpose of war is "is to persuade the other side to lay down arms" (War has many purposes only some of which are present in any given conflict, this is a rare one mainly reserved for internal conflicts, and there has even been a war with the aim of persuading the other side to retake up arms).
To clarify my (my twin's) position, their were 3 possibly successful options the US could have pursued in Afghanistan 1. change the goals so that it doesn't need to remain in the country, say to just removing the Taliban and Al-Qaeda, this is the best option in my opinion 2. going in with the willingness to commit massive genocide, by this I mean willing to kill 10-50-90% of the Afghan population (The Roman option) 3. Sending in sufficient troops to actually conquer Afghanistan (The Tibet option) other options existed and their were ones for Iraq. The US chose none of these as they were either not willing to pay the prices, which would have been both immaterial and material, and their pride mixed in with the neoconservative vision. This has lead to a situation where American is simultaneously using both too much and too little force. Additionally there is a small chance we will see Afghanistan conquered in the near future by a Russian-Chinese joint effort to contain Muslim extremism.
Also just as a general statement against those who say that war is 'senseless' and futile. War is often rational from a logical and societal perspective while making perfect sense. It is perpetrated by ordinary people and often the aims of one side are achieved, note these aims are not noble in most senses (except gaining self-determination, see Vietnam) but that doesn't particularly matter to whether they're achieved. The way to foster peace is not to denounce war or scapegoat but make it so that situations where it is the logical choice are rarer. And the idea "those who make war never have to fight war" is incredibly stupid, it only relates to recent societies. Roman consuls (their presidents) fought on the front line, so did the Roman emperors, in fact to have a political career in Rome one had to already have served in the military, Napoleon was shot at while studying war up close , most medieval rulers and the equivalents in other human societies did the same. Yet they didn't have any shortage of wars and as established the world now is relatively speaking at peace when our leaders don't fight on the front line.
The industrial capability of the eco-technic societies of the future could easily fight massive wars that are the equal of WWI or WWII (at least proportionally speaking), along with many smaller ones. In the shorter term, as nuclear weapons slowly disappear and become relics of a deceased age, their ability to act as a deterrence will degrade and other powers will slowly nudge the boundaries of what's possible before they're nuked upwards. Eventually all they'll stop is total-war, and eventually even that capability will disappear. Total war, similar to the world wars will be fought on an industrial scale by the eco-technic societies of the future, but that's after overshoot has taken place and a recovery has begun, before that we can expect wars to return to what they once were, smaller scale and mostly over peripheral areas. The destructiveness of war won't however return to what it is in tribal societies, 1% of the population won't die every year because of wars and there will be large chunks of time in which there are no wars (at least in a given region), unlike in tribal societies in which war is a constant and every where.