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Nuclear war, maybe. WWIII would require two global superpowers, not a global superpowers and a country with the GDP of Greece that's struggling to remain a regional power.

Yeah, OK, though a nuclear war waged over 2 days would likely cause more damage than WW2 + WW1 combined given the population density and destroyed economical value.

> WWIII would require two global superpowers,

No, it wouldn’t. You can have a World War without global superpowers on both sides (you need a wide geographic alignment of such power to, balanced for the relative difficulty of force projection on both sides, reach aggregate near-parity across a widely geographically dispersed set of conflict theaters, but you can do that with a global superpower on one side and a coalition of major regional powers in different regions on the other.

Entirely depends on where China sits in this discussion.

Luckily despite the last ten years of decline in relations and shift in China's positioning, the PRC still seems mostly motivated by trade rather than dick-measuring contests. Let's hope that continues.

So it's NATO against Russia, Belarus and who?

Should it expand beyond a major European war: Iran, Syria, North Korea, China are among the more obvious potential out-of-region Russian coalition partners; there's also quite a number of situations in Africa that could also be plausible areas of expansion of the same geopolitical conflict.