Yep, because of nuclear MAD.I don't think any president from the nuclear age (Truman through Biden) would have sent American troops to Ukraine and risk a nuclear war. Send money and support, sure, but there is no history of direct military engagement with Russia or the USSR, despite them doing plenty of things like this over the years (Turkey/Greece with Truman, Eastern Europe in the 1950s/60s, Afghanistan with Carter and then Reagan, Georgia with Bush, Crimea with Obama, Syria with Trump, etc.)