The US has to retaliate, as without doing so, it will hand world-domination to Russia, China and any two-bit dictatorship willing to use nukes. Moreover, Russia has repeatedly threatened European NATO members, and those members would rather the battle be fought in Ukraine than on their own turf.
Strawman. Ukraine is not in NATO and is thus forced to keep neighboring Russia happy. That's just realpolitik. US can not and will not retaliate as that would be WW3. On the flip side, Russia will not attack NATO countries as that would also be WW3.
The only real losers here are primarily Ukraine which is being destroyed and Europe which has to keep doing what US wants since it's dependent on the latter for defense.
Your thinking in this area lacks depth. Think about what happens next after Russia is handed Ukraine. And after that.
And yet USA has had no issue fighting against the soviets during Vietnam and Korean war.
You can try and be armchair president all day long but the truth doesn't favor your argument.
Ukraine is not in NATO which means that the Russians have and will maintain escalation dominance. They can detonate nuclear weapons in Ukraine without immediately triggering Article 5 guarantees. I presume that if that takes place, the US will find it very hard to retaliate.
That would be a colossal disaster for Ukraine but also horrible for Europe, and yet, this is where continuous escalation leads to.