Ukraine isn't a major power, and the fact that they've been able to hold back Russia so effectively, with so few resources, is mainly an indicator of how weak Russia is these days.
There's been plenty of war, and it could well be the first step on a path to the end of life on earth. I imagine this belief stems from all the "Hiroshima and Nagasaki were necessary/good" propaganda.
The problem with game theory is that the actors do act rationally, and the most rational scenario is still the doomsday genocide: first strike, with a submarine counterstrike.
Full scale war between Russia and Ukraine (with help of NATO) progressing right now with constant threats to use nuclear weapons or blow up a nuclear power station.
There hasn’t been a full scale war between major powers since.