To the contrary, retaliation is game-theoretically sound. You can't allow an adversary to take what they want and win every battle "because they have nuclear weapons" without capitulating world domination to that adversary. This is like playing poker and folding every hand--you will quickly lose everything. World War 2, and the Sudetenland proved this very well--you can't forestall war by gifting land to greedy dictators, it just makes them hungrier.

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.