In the modern world, World War III is a near impossibility, and the Cold War has shown us that Mutually Assured Destruction prevents a large war between super powers from occurring. Sure there have been small conflicts over recent years, but no World War III.
Is it possible for World War III to occur and not have it devolve into nuclear war? If so, how do I prevent nuclear war in World War III?
The crux of the question is preventing nuclear attack/nuclear exchange rather than preventing WW III. By some counts we passed WWIII a long time ago:
Seven Years War: First global war with theatres of operation in both hemispheres (WWI)
Napoleonic Wars (WWII)
Great War of 1914-1918 (WWIII)
War of 1939-45 (WWIV)
Cold War (WWV)
Global War against Terrorism (GWOT) WWVI
This would make a subsequent global conflict with different actors WWVII
Nuclear weapons have not been used since 1945 since the threat of Mutually Assured Destruction have kept the Great Powers from using nuclear attack, but instead, warfare has evolved to allow for conflicts to remain below the threshold of nuclear weapons release. Indeed, the imbalance of power with non nuclear weapons between First and Third world societies has become so great that warfare has moved to what some theorists call 4GW. The general definition that I like to use is:
Fourth-generation warfare (4GW) uses all available networks — political, economic, social, and military — to convince the enemy’s political decision makers that their strategic goals are either unachievable or too costly for the perceived benefit. It is an evolved form of insurgency. Still rooted in the fundamental precept that superior political will, when properly employed, can defeat greater economic and military power, 4GW makes use of society’s networks to carry on its fight. Unlike previous generations of warfare, it does not attempt to win by defeating the enemy’s military forces. Instead, via the networks, it directly attacks the minds of enemy decision makers to destroy the enemy’s political will. Fourth-generation wars are lengthy — measured in decades rather than months or years.
(The Sling and the Stone': Next-Generation War by Col. Thomas Xavier Hammes (USMC Ret.))
By this measure, using nuclear weapons is counterproductive for the most part. Nuclear terror attacks made by smuggling in nuclear devices may serve to convince decision makers that the conflict is no longer winnable, but human psychology and historical examples like the Blitz demonstrate attacks like this could serve to harden the resolve of the nation instead.
If nuclear weapons are to be used in a conflict, it would be for very specialized purposes where the release of a nuclear weapon does not raise the threshold of weapons release on the ground. Some people suspect that tactical nuclear weapons used at sea might pass this test. Nuking an aircraft carrier in the Pacific does not imply or demand a city or land base target be struck by a nuclear weapon in response.
Space is another place nuclear weapons might be used freely. The high energy density of nuclear weapons allows you to carry out conflicts in space even with the enormous distances and speeds involved. Atomic Rockets has a page describing the use of nuclear devices to power kinetic energy weapons, ranging from shotgun charges shooting pellets at 100km/sec, shaped charge nuclear warheads shooting streams of liquid metal at .03 c and even Casaba Howitzers shooting streams of high energy plasma at .1 c. Nuclear weapons can also energize X-ray lasers.
In theory, such weapons can be used against ground targets from orbit, but the political authorities will be very wary of getting to that point, since it will be too close to an actual nuclear attack.
For the most part, I would see WWVI and WWVII to be much more subtle conflicts using deception, economics, propaganda, assassination, cyberwar and other tools to destabilize and demoralize an enemy (much like Russia did to Ukraine since 2014), meaning the actual use of forces can be devolved to supporting local malcontents, the deniable use of SoF units and the occasional incursion of conventional forces or artillery to make sure a difficult objective gets taken.
Studying how the Russians operate in Crimea and Eastern Ukraine, or the Chinese in the South China Sea and you will get an idea of how different large scale conflicts between major powers could be in the future.
If either side has an effective Strategic Defense Initiative in place then conventional nuclear weapons will be useless against them. This could be a extensive system of ground or space-based anti-ballistic missiles, high-velocity rail guns, or anti-missile lasers capable of defeating even the largest of nuclear attacks with minimal leakage. The technology for this isn't too far off as all 3 technologies exist and are currently in use in the military. If a WW3 was impending one imagines that fortifying one's country with such systems would become a priority.
Nuclear weapons would likely still be utilized on the front lines in various capacities, but we would not see a nuclear holocaust.
Here's a weird answer: over decades, all the nuclear powers are facing increased tension, slowly developing. Over the years, they gather to negotiate more monitoring of each others' systems, and each side implements more safeguards against accidental launches. The day comes that they go to war. Eventually, one side decides to launch, only to discover they've created such a beaurocratic and technical knot that no chain of command actually has the ability to issue the launch orders. They've given so much over to computer monitoring and that requires approval in triplicate to access that the missiles are totally inaccessible. The generals and techies may not have intended this result, but it just sort of happened because it was impossible to test an actual intention to launch. The only time they ever did practice runs was with the computers in test mode, which apparently works completely different from live fire mode, but no one realized that bug had been introduced. Digging the missiles out of their secure bunkers or creating a new control system will take longer than the war's duration.
There's a lot of detail to work out around how the situation arose (and whether it was intentionally done by a clever diplomat putting specific requirements into various treaties), but I think it is a viable option.
Mutual Assured Destruction means that nuclear weapons are essentially a weapon of last resort: a way of saying "If I can't win, then neither can you".
If you want WWIII to be non-nuclear, then you need to avoid situations where one side or another would be tempted to use them.
Basically, a non-nuclear WWIII would consist of an American superpower and a Eurasian or African superpower fighting, where most of the combat takes place in third-party territory (eg. Pacific islands), and ending when the losing side has been pushed back to its core territory and clearly can't continue fighting, but the winning side just as clearly isn't in a position to mount an invasion.
(Another option would be North American vs. South American or Eurasian vs. African, with back-and-forth fighting settling down to a stalemate at a geographic chokepoint.)
I would like to expand on the last paragraph of Thucydides' already excellent answer. I hope this will allow you to understand better that the use of Nuclear Weapon in the next 'global war' is an unlikely as ever.
The Russian operations in the Ukraine is a great example of how wars will be in the future. The days of countries openly and formally declaring war on each other and then proceed to prosecute that war with all the limitations imposed on the belligerents by any Laws of War such as the Geneva Conventions or things like the Law of Land Warfare is fast coming to an end. This is mostly due to nation states' realizations that they have a lot more strategic and tactical flexibility if they engage in an undeclared hybrid war such as the one the Russian Federation is prosecuting in the Ukraine, Iran is prosecuting in Syria, and others. (For a comprehensive study of how Russia prosecutes this war please read this highly interesting paper by the Potomac Foundation.)
'Hybrid Warfare' or 'Non-Linear Warfare' or the Russian 'New Generation Warfare' is of course a term first popularized by GEN Valery Gerasimov, the CoS of the Russian Armed Forces in a paper titled The Value of Science in Prediction(linked article in Russian. Here's a good translation.) According to Gerasimov:
In the 21st century we have seen a tendency toward blurring the lines between the states of war and peace. Wars are no longer declared and, having begun, proceed according to an unfamiliar template.
He then continued to press his point in the most interesting, and important, paragraph of the article:
The role of nonmilitary means of achieving political and strategic goals has grown, and, in many cases, they have exceeded the power of force of weapons in their effectiveness.All this is supplemented by military means of a concealed character, including carrying out actions of informational conflict and the actions of special-operations forces. The open use of forces — often under the guise of peacekeeping and crisis regulation — is resorted to only at a certain stage, primarily for the achievement of final success in the conflict.
This means everything from information warfare (agit/prop), the use of proxies (rebel groups, militia, irregular forces), the use of undeclared SOF (see Little Green Men), economic warfare, the intermixing of GPF/SOF personnel in proxy groups (declared as 'volunteers'), the support of proxy groups by high-end kinetic capabilities (cross border artillery, UAVs, etc).
The war in the Ukraine, Iraq, and Syria are nasty conflicts without a doubt. Anything from massed fire into civilian areas to the summary execution of wounded combatants were seen. What we do not see, however, is the use of nuclear weapons.
There are basically four reasons why your country needs to deploy nuclear weapons:
All these are nice and good if you're engaged in an open, conventional war with an adversary state, where combatants of both sides wear uniforms, acting under legal orders, and are bound by the Geneva Conventions. In a New Generation War? Not so much.
The use of nuclear weapons are always a double edged sword. Yes, its massive destructive power will obliterate anything you hit. But on the other hand, anything it hits is no longer viable for extended operations. Even if your armies can operate in MOPP, they would not be able to prosecute effective combat operations for long. Their MOPP suits will quickly degrade their performance, use up supplies, and make the area unsuitable for staging.
Moreover, it will alienate the using power from the population it seeks to either win over or cow, quickly limiting its ability to operate freely in its AO, making your armies devote too much time to Force Protection instead of offensive operations.
It will also break the shield of deniability that the user has projected to hide their operations. Russia can deny the Little Green Men were theirs (until, of course, they admitted it), but if all of a sudden accurate, massed nuclear fire hit Ukrainian Army positions - well, you can't hide and deny that.
So to answer your question: The way WW3 happens on your world without the use of nuclear weapons is to realistically portray the conflict as a series of interconnected low-intensity hybrid warfare prosecuted by several different governments, their irregular proxies, and various other non-state actors (Al Qaida, IS/Daesh, Boko Haram, Abu Sayyaf, etc) that take place around the globe at roughly the same time for a decade or even more. Its close enough to what we are experiencing right now that you can make the scenario very realistic.
To avoid a full-scale nuclear war, all you need to do is eliminate most of the people who know the codes which can launch the missiles. Any reasonable airborne pandemic can do that.
Release a bio-weapon to knock off 40% of the planetary population. This will destroy most of our infrastructure and take out 4 out of every 10 people who know the codes.
Then, as the millions of un-buried corpses rot and putrefy, additional diseases will ferment, killing off more of the population and contaminating the food and water supplies. Within a few months, only 10% of the original population will still be alive. With any luck, only 1 in 10 of the original code holders will make it this far.
With so few survivors left alive, the few who know any of the launch codes will be too busy surviving to use them. Within a generation or two, those codes will be ancient history and the missiles will be a bunch of rusted relics.
Now your growing population can get back to the time-honored art of world-wide warfare, without the threat of nuclear annihilation.
