In the US, justifications for the war often depend on a contemporary domino theory: unless we stop Putin in Ukraine, he won't stop there. But the time has come for Europe to stand on its own feet. That doesn't mean it has to stand alone, but it must not continue to use America as a crutch.
Notes from Wikiquote
The Domino theory during the Cold War postulated that if one country came under the influence of Communism, its neighbors would follow. It was used as an argument in favor of American intervention.