Nuclear Weapons
Most of the nuclear weapons in the world are H-bombs (which create a nuclear fusion explosion from heavy hydrogen) and ICBMs (Intercontinental Ballistic Missiles that carry H-bombs) with multiple warheads that spread out over a large area, and a much greater explosive power than the Hiroshima and Nagasaki A-bombs (that created a nuclear fission explosion from uranium) that were the first and only time that nuclear weapons were used in combat in 80 years ago.
The problem with one of these typical weapons in nuclear arsenals worldwide is that blowing up everyone and everything in a major metropolitan area, irradiating that area for many years, doing significant harm to the global environmental, and risking a counterattack in kind from other nuclear powers under the Mutual Assured Destruction (MAD) doctrine, is almost never a militarily useful thing to do. We justify them primarily under the MAD doctrine as a way to discouraging anyone from ever using nuclear weapons.
Even for extreme military missions like "bomb Mecca", or destroying the headquarters of an enemy government in a capital where its top leaders are located, or destroying a large military base isolated from civilian populations, or penetrating deep bunkers (such as a North Korean or Iranian nuclear facility or an ICBM missile base or an underground submarine base in China), a single warhead much less potent than a typical U.S. ICBM or SLBM (submarine launched ballistic missile) and closer in explosive power to the Hiroshima and Nagasaki A-bombs, are more than adequate in terms of explosive power.
Really, the only mission for which such powerful weapons make any military sense is planetary defense, to break up a large incoming meteor or comet into small enough pieces to burn up in the atmosphere before doing serious damage, or to divert an incoming meteor or comet from a collision course with Earth, in the basically oxygen free environment of deep space.
Indeed, one of the main reasons that nuclear weapons haven't been used again for 80 years, despite the fact that quite a few countries now have them, is precisely because they aren't a good fit to any military objectives, and not because the countries that have them been particularly ethical.
Tactical nuclear weapons, such as the backpack sized "Davy Crocket" bomb from Cold War era, are another matter. These can be as potent as, or perhaps ten times as potent as the largest conventional missiles and bombs, but can be five hundred times less massive. This makes them the right size to bust deep bunkers, sink aircraft carriers and other large warships, or destroy a large dam, bridge, or military base, or a dictator's palace while the dictator and his top lieutenants are in residence, with a single blow. This could be delivered using a small hypersonic missile, or small fast supercavitating torpedo, or small stealthy drone, or a guided bomb delivered with a stealth warplane, or by special forces soldiers disguised in a civilian boat or car or ATV, that can more easily evade air defenses that could more easily intercept a conventional sized missile or conventional bomber aircraft. If one could make a tactical nuclear weapon with a much smaller yield than even the 10-20 kiloton Davy Crocket bomb, one would even use it to make much smaller anti-armor/anti-ship/anti-fortifications weapons - with a four to ten pound mini-missile doing the damage of the two thousand pound bombs that are the largest typically used by fighter aircraft and as conventional ship and submarine launched missiles.
But, the much smaller size of the bomb is less likely to provoke a civilization ending counterattack, and, particularly if it is an H-bomb that doesn't spread uranium or plutonium all over the place, leaves a much smaller area irradiated for a shorter time period and doesn't do nearly as much damage to the global environment.
What this means for policy is another matter.
On one hand, it suggests that investing in large numbers of multiple kinds of large modernized nuclear missiles and bombs doesn't really make sense. Optimally, one wants no more of them than is necessary to present a credible MAD deterrent. Why, for example, should you have land based ICBMs at all, when they turn those bases into first priority targets in World War III and are easier to use missile defenses against than SLBMs and nuclear bombs delivered by stealth aircraft or shorter range hypersonic missiles launched from aircraft.
On the other hand, if you create a significant arsenal of tactical nuclear weapons, particularly if other nations and insurgent groups follow your example, it is much more likely that they will actually be used in war. And, honestly, existing conventional bombs are more than adequate to destroy or disable that most heavily armored vehicles and all but the deepest and most secure bunkers, without crossing the line of normalizing the use of nuclear weapons in war.
Nuclear Batteries
Nuclear power for military applications has only been used for military submarines and large ships like aircraft carriers and mega-cruisers, in part, because they have been hard to scale down in size, and in part, because the nuclear materials in them could be used by an enemy, if captured, to make nuclear weapons.
But nuclear batteries, using lighter radioactive isotopes than uranium and plutonium (like thorium), that are not nearly as suitable for use in nuclear weapons, share the immense energy density of existing nuclear power plants on submarines (which use only 10 kg of fuel for decades), and large warships, and can be scaled down to arbitrarily small sizes (e.g. a pacemaker or watch or single automobile). Nuclear waste from nuclear batteries using lighter isotopes is also reputedly easier to dispose of than uranium. This has tremendous (I hesitate to use this overused phrase but it's appropriate here) game changing potential to reduce the logistics burden of military units.
In fiction, this is basically the innovation that makes Iron Man's suit possible.
Nuclear batteries facilitate quieter, lower maintenance, all electric military systems that don't need to be refueled for years, and that can also use that spare power to recharge electric devices in the unit and military lasers and directed energy weapons, and they allow fuel supplies to be decentralized to individual vehicles, systems, and equipment items. It generates no smoke or emissions smells. Combined with night vision, IR sensors, and LIDAR these vehicles could operate without lights at night. If a ship with a nuclear battery were sunk, it wouldn't create an oil slick that would endanger sailors who abandoned ship.
The logistics burden involved in providing water to troops could be reduced as well. In coastal areas and at sea, it could power a desalinator, at least for showers and cleaning use, even if it wasn't good enough to make tasty water for drinking. In deserts, it could power a device to extract clean water from the air. In jungles and forests, it could power a water purifier that would allow local water supplies to be used. It could also power heat pumps in places that were too hot or too cold.
No more tanker ships. Propeller and rotor driven aircraft could dispense with aerial refueling tankers. No more vulnerable, unarmored convoys carrying diesel fuel. No more vulnerable fuel depots. No more reliance of distant fuel supplies whose prices are influenced by foreign powers who may become adverse to you.
And, because nuclear batteries can be much more compact than conventional fossil fuel tanks or chemical batteries, without sacrificing range, there is one less thing in the military system which can be easily struck causing the vehicle or military system to explode from its own fuel, because nuclear batteries are a much smaller target and, if properly designed with this risk in mind, aren't necessarily explosive if hit with a high explosive round.
Things That Are Hard To Predict
The mix of drones (armed and unarmed), guided bomb and missiles, active defenses including but not limited to directed energy weapons and jammers, new materials that could make armor lighter and stronger, hypersonic missiles, prototype "invisibility" cloaks, claimed (but not demonstrated) sensors that can penetrate radar stealth, and the use of artificial intelligence to organize information in the fog of war, identify incoming targets and respond before a human could react, and guide drones and guided munitions without active direction from a human controller and guide sniper weapons more accurate, all combine to make the future of warfare hard to predict.
Active defenses, particularly as used by Ukraine and Israel in recent conflicts, and by U.S. naval ships, have made securing air superiority much more challenging, intercept something like 85-90% of incoming missiles and drones reversing the one shot, one kill trend established by the first guided weapons, and could be the only thing that rescues tanks and fixed forward operating bases from obsolescence (while allowing the use of less armor since threats only countered by heavier armor can be actively intercepted).
And, these are only in the early days. There is plenty of room to make interceptor munitions less expensive (particularly with directed energy weapons), to make them and their associated sensors smaller, lighter, and more secure, to increase the percentage of incoming munitions that are intercepted, and to increase their range.
On the other hand, countermeasures to active defenses, like AI guided and wire guided suicide drones that make jamming ineffective, are being developed rapidly too, out of necessity. So far, suicide drones have been mostly crude modification of commercial, off the shelf quadcopters, but there is also plenty of room to make them quieter, harder to see, and to make them harder to discern with radar and electronic signals.
Part of what makes the future course of these technologies so hard to predict is the constant, rapid Red Queen hypothesis evolution of measures and countermeasures.
For example, recent efforts to develop rail guns with unguided rounds that rely purely on kinetic energy without resort to explosives or guidance systems, which were set aside because they were too difficult technologically, may receive renewed attention for their immunity to active defenses based upon jamming, lasers, or other directed energy weapons (like microwaves), and may be possible to scale down in size as heavier armor is abandoned in favor of active defenses.
Armed drones are much smaller and cheaper than comparable manned warplanes, warships, submarines, and military ground vehicles, and can be used to take much greater risks since the smaller ones are expendable. They have become the dominant weapons in the Ukraine War and in military encounters directly between Israel and Iran, and have been one of the primary weapons used in the war on terrorism carried out by the U.S. in the wake of 9/11. They can strike targets well beyond the range of slug throwing artillery, and can be effective in precisely striking the most vulnerable parts of moving targets whose exact location isn't known when they are launched, while reducing the risk of friendly fire.
So far, simple, remote controlled, flying drones have been predominant in military applications. But, we have seen glimpses of other possibilities. Large sophisticated drone fighter aircraft are smaller, cheaper, and capable of higher-G maneuvers and faster reaction times than manned fighter aircraft. Flying drones have been used to resupply troops in war zones in environments where there is too much enemy fire or the terrain is too difficult for conventional logistics convoys. Drone jet skis and speed boats filled with explosives have destroyed large warships in the Black Sea. Small, comparatively inexpensive, drone ships carrying anti-ship missiles in shipping containers have been successfully tested against target test ships. Small anti-personnel aerial drones carrying small arms or grenade sized munitions have been prototyped but not yet widely used, but could facilitate sniper-like tactics in conditions where these tactics were previously not viable.
Reconnaissance drones can identify enemy forces in ways that previously took much larger and more expensive helicopters, fixed wing reconnaissance aircraft, and satellites, at a cost and size that make them cost effective for even platoon or squad sized army units to use organically for themselves in real time. Coast guard cutters can use them to patrol far more of the sea at once, and more quickly and in more places at once, than any ship or boat could. Smaller battery powered reconnaissance drones can be similar in size, speed, and noise level to birds, making them harder to detect. The smallest insect-sized reconnaissance drones can enter buildings or clear anti-drone netting, undetected, aren't that expensive, are harder to shoot down if detected, and can be deployed in swarms. There have also been successful efforts to put cameras on actual roaches and other insects for tasks like locating survivors in the rubble of collapsed buildings. Small remotely monitored sensors can constantly monitor roads, bridges, fields, buildings, and ports with a much reduced risk of detection. And, unlike a human scout or forward observer, who is also easier to detect, they can't reveal many secrets, or create a POW hostage situation or casualty if they are captured.
Submarine drones can be on constant patrol for days or weeks for enemy armed submarines, sea mines, and enemy ships, or could follow enemy submarines to their hidden bases, while appearing to sensors like sea life. Submarine drones or mostly submerged drone ships can be used to smuggle supplies in interdicted waters, in the way that drug dealers have used them. Submarine suicide drones (or armed submarine drones) can be used to destroy enemy ships without being detected until it is too late, at distances far exceeding what is possible with a conventional torpedo.
Nuclear batteries could be used to create drones or remote sensors that can operate autonomously for years without human contact or maintenance.
It is hard to know what kind of balance and mix of technologies will emerge as experience and economics guide the choices that make sense as these technologies become more mature.
For example, lots of drone and missile guidance systems (and military aircraft) are expensive to a significant extent, not because they are expensive to manufacture, but because a large premium is being paid for the intellectual property rights arising from the development process. So, a significant part of the cost barrier to their use is economic and could be addressed without any technological advancement (perhaps the military would be better off paying for R&D itself on a work for hire basis and retaining all of the intellectual property).
Things We Can Predict
This said, there are some predictions and conclusions that can be reached with some comfort:
* Tanks and conventional warships without active defenses are as obsolete against a near peer opponent as horse cavalry, warships with sails and cannons, swords, and bayonets. Howitzers and mortars throwing unguided slugs will follow soon.
* Large, mostly unarmored and undefended forward operating bases near "front lines" (such as they are), massed formations of ground troops and armored vehicles, and small pillbox type fortifications are obsolete tactics against near peer opponents.
* No armor, unaccompanied by active defenses, is effective against anti-armor weapons, which are increasingly inexpensive, light, and effective, making large, heavy military systems that rely on armor ineffective against near peer opponents. Armor pretty much only makes sense against enemy forces or insurgents or civilians with only small arms.
* The cost and availability of the weapons necessary to be effective, in a cost-effective way, against a big budget sovereign nation military force has greatly declined.
* National borders, national waters, and military tactics organized around front lines have limited relevance when long range strikes are widely available.
* Large nuclear weapons will continue to not be used because they don't serve useful military objectives.
* Biological weapons are unlikely to be widely used until they can be better controlled, because the risk of blow back to the user is great.
* Despite being lumped in with nuclear and biological weapons, chemical weapons, at least historically, have not been significantly more effective than conventional weapons, and have been amenable to effective countermeasures when they have been used like air sealed vehicles with positive air pressure, filters, protective clothing, and gas masks.
* Advances in modern military weapons have made great strides in destroying distant and armored targets, but drones, missiles, and guided bombs can't hold or control populated territory, which is one of the most common military objectives, without boots on the ground.
