Another thorny issue will involve U.S. economic sanctions. In any talks over ending the war, Russia is certain to demand that sanctions be lifted. U.S. officials have said that sanctions relief will be up for discussion, but only after Russian troops are out of Ukraine.

“But that was before Bucha,” Daalder said. “The atrocities have made it much more difficult to contemplate lifting any sanctions.”

If a cease-fire takes hold, the United States will also be expected to play a leading role in helping Ukraine rebuild its armed forces, its economy and its shattered cities.

U.S. and allied support after the war will be just as important as it is now. Putin has invaded Ukraine three times since 2014; if his current offensive falls short, he can be expected to plan a fourth. One goal for U.S. policy should be to deter him from trying again.