GJD said:
There is still a Twilight in 2300AD, so whilst not technically a 3rd world war (or maybe it is) there is still a 100 year or so mini dark age.
Also, Alcubierre warp drives have a ton of problems with them (not the least of which is potentially requiring a mass of exotic matter - material which as far as we know does not and can not exist - greater than the mass of the observable universe). THe stutterwarp is a core component of what makes 2300AD the game it is. It's effects and limitations are well known and are integral to the background. Whilst you could easily make a hard scifi game without Stutterwarp, it wouldn't be 2300AD.
G.
There is no Soviet Union, who would we go to war with? Wars are started by people who think they can win, so who would think they could win World War III and conquer the World? Also why would they think that? The Soviet balloon has been popped, there is no putting it back together. I don't think Russia would do it anytime in the near future, they know if they launch those missiles, they are going to be dead, they can create a World War III disaster by launching those missiles, but they have no motivation to do it, because they'd know they would lose by being dead along with their enemies, and Russia is a weak power anyway, it is weaker that Germany, with an economy the size of New York State.
Lets look at China, would they start World War III? China's economy depends heavily on exports, what benefit would it be to them to launch World War III? They can't conquer the World, even if they built up their conventional forces, and some point, someone with nuclear weapons would stop them, and when that happened, China would be a lot worse off than before they started, it might even be dead or broken up. I can't see the Chinese leadership wishing that outcome on themselves, and any outcome where they win would require that the rest of the World allow themselves to be conquered without using nuclear weapons
There is North Korea, but it doesn't have enough nuclear weapons to destroy civilization, but the World would destroy them in response. I also can't see Iran starting World War III.
There is another possibility, but I'm not sure you'd want to use it. The Terminator scenario. You remember the Terminator movies, when Skynet seized control of our nuclear arsenal to star World War III in order specifically to wipe out humanity? I'm not sure you'd want to go with that scenario. It would require advanced Artificial Intelligence.
Another possibility is that our Sun gets really nasty, and erupts a solar flare which destroys our power grid, fries our electronics, cars don't start, cities go dark, millions of people starve as the transportation networks can no longer deliver their supply of food. The population rapidly declines due to starvation and disease before enough hardware is built to restart our 21st century civilization with fewer people, maybe one billion in total with a lot of empty cities. Are we going with that scenario?