Post-Vietnam the US hasn’t engaged in large scale active combat against communism or Russian-backed forces. It has instead favored selling weapons to Eastern European states and building the NATO alliance.
Putin has been clear about the breakup of the USSR being a mistake in his eyes. He’s been annexing territories (or in his eyes, re-annexing) since 1999. He looks for countries with strong Russian cultural and linguistic backgrounds and props up allies that will take over. If that doesn’t work he ratchets up the pressure until he feels comfortable using military force. See: Chechnya, Georgia, Crimea and now Ukraine.
The US is far stronger militarily than Russia, but with nuclear weapons in the equation, no one wins.
It’s easy to be an arm chair hawk and say we should go fight, but it’s another thing to look you kids in the eye and explain they’re going to die from radiation fallout.
Instead, the US will likely prop up the resistance once Ukraine falls and try to make occupation as costly as possible just like we did in Afghanistan.