These factors make it possible to envision a very different series of events over the past few weeks. Once Putin’s mobilization inside Russia began, a Western coalition could have sent troops to Ukraine. “He who wants peace must prepare for war,” Evelyn Farkas, a Pentagon official in the Obama administration, wrote in January, calling for a 1990-style coalition. “Only a balance of military power — a deterrent force and the political will to match — can keep war at bay.”

“Putin is someone who responds to brute force,” Ian Brzezinski of the Atlantic Council told The Times before the invasion.

Yes, such a showdown would have carried big risks. Confronting a nuclear power is not easy. But there is a long history of successfully doing so, dating to the Cold War. (Otherwise, any country with a nuclear weapon could simply annex any country without one.) And of course the lack of a military response also carried big risks — which have now turned into terrible costs.