Russia could also intensify attacks near the North Atlantic Treaty Organization’s borders with western Ukraine or threaten nearby NATO countries militarily to sever the lifeline keeping Ukraine alive. Putin previewed this tactic when he used cruise missiles to pummel a base near the Polish border formerly used by U.S. troops — and now used by foreign fighters flocking to aid Ukraine.

Any of these options could appeal to a dictator looking to win a fight that, as U.S. Director of National Intelligence Avril Haines has said, he “cannot afford to lose.” And any of them could provoke louder calls for intervention in the West.

The U.S. could conceivably have to reconsider its position even short of deliberate Russian escalation. Suppose a stalemated war drags on for months or years, producing terrible bloodshed within Ukraine and instability and insecurity beyond it. Suppose scared, frustrated Russian forces respond to dogged resistance by murdering civilians.

In the 1990s, Western democracies ultimately intervened in the former Yugoslavia to end a conflict that was having similar consequences. Today, writes recently retired Australian general Mick Ryan, “there may be a requirement for a military intervention” in Ukraine, “if the west doesn’t want a forever war on the doorstep of Europe.”