The US was in a pickle thanks to the deal Clinton struck with Russia after the fall of the USSR when the nukes in Ukraine were a whole deal. If Putin had been smart he would have maintained the Crimean-occupied status quo but he has talked for years about returning Russia to its former glory and his pals have run with this. Hell some of them have been talking for months about "taking back Alaska" which I mean is absolutely hilarious given the sheer amount of firepower stationed there.

Russia's preventative war has ended up draining it of a ridiculous amount of men and military hardware. Not only that but they managed to make what was thought to not be that great of a system in the Patriot System look out of the world fantastic. He might end up "winning" the war and getting the lands he has already taken but Russia is having a huge population drop and while the same can be said about Ukraine those citizens have said they would return once the hostilities end. No one wants to go back to Russia.