A couple key things to consider:
1. Ukraine is referred to as the “Invasion Highway” historically due to its geography. It is the fastest route to invade Europe (from the east) or to invade Russia’s capital (from the west). For that reason, geopolitically, Russia cannot afford to compromise on Ukraine and allow it into NATO. This is worth fighting for from Russia’s perspective. I’m not so sure Russia has ambitions on taking back the other states that used to be a part of the USSR – just those that are geopolitical buffer zones (keep in mind the US policy of creating a ring around Russia and China to enclose them).
2. When you think of how easily wars have begun throughout history, it’s not hard to see how a war could start now. If you listen to Jim Rickards (highly recommended) there is an economic war going on as we speak, which can spiral into a physical conflict.
3. Wars may not be fought “traditionally” with planes and large marching armies, but guerilla-style – especially with the prospect of overwhelming force against them (the Russians). The US can be broken without a takedown of the military/airforce/navy.