Mr. Laser Beam
, you're mistaking one possible form
of WWIII for the only possible form. As I've said, many works of fiction portray WWIII as a limited
nuclear exchange. Not all nuclear weapons are huge multi-megaton city-killers; there are also smaller, tactical nuclear weapons designed for more surgical strikes. Both fiction and real-life military analyses have always acknowledged the possibility of a more limited, targeted nuclear war that would fall far short of global armageddon.
Keep in mind, after all, that the bombs dropped on Hiroshima and Nagasaki did not actually do more
damage than the most extensive conventional firebombings employed in the war. They were just a more efficient way of doing what had already been done to the majority of cities in Japan by that point, as well as many in Europe. So nuclear weapons are not automatically
a quantum leap upward in destructive scope. They can be more targeted, focused, efficient.
And this makes perfect sense in a Trek context extrapolated from our own present state. The idea that nuclear war would destroy all life on Earth was a product of the '70s and '80s when the US and USSR built up hugely disproportionate nuclear arsenals, driven as much by the greed of defense contractors as by any legitimate military need for that much overkill. But we've both disarmed significantly since then. Not to mention that the geopolitical status quo has changed considerably since the old two-rival-superpowers paradigm; there are more nuclear players, most of whom have smaller arsenals and more localized rivalries. So the old all-or-nothing paradigm of WWIII that you're thinking of is outdated. There are many possible ways it could go. A nuclear conflict could devastate some parts of the world while leaving others virtually untouched. It could be asymmetrical, with one nuclear state devastating a non-nuclear state so that there'd be no mutually assured destruction. Or it could be fought surgically, with small, tactical nukes directed mainly against military targets. Heck, thinking in modern terms, it might be fought mainly using suitcase nukes smuggled in by suicide bombers, rather than by spreads of missiles flying between continents.