> Something I'm unsure about is what does and doesn't trigger wars.
According to Clausewitz, "war is the continuation of politics through other means." A state goes to war when it believes it can achieve a political objective through force that is infeasible through diplomacy, deterrence, soft power, intimidation, etc.
The US and Russia fighting directly would be quite dangerous due to the nuclear capabilities of each, so whatever political objectives America might wish to achieve through a hot war would be (quickly) overshadowed. Thus the US has revived its Cold War stance of supporting Russia's enemies without direct involvement.
According to Clausewitz, "war is the continuation of politics through other means." A state goes to war when it believes it can achieve a political objective through force that is infeasible through diplomacy, deterrence, soft power, intimidation, etc.
The US and Russia fighting directly would be quite dangerous due to the nuclear capabilities of each, so whatever political objectives America might wish to achieve through a hot war would be (quickly) overshadowed. Thus the US has revived its Cold War stance of supporting Russia's enemies without direct involvement.