As Biden has said, if Russia even tiptoes over the line into NATO territory, we’ll have little choice but to take up arms directly. The same might conceivably be true if Putin unleashes chemical weapons — an option for which he appears to be laying a false pretext.

But absent that kind of broader menace, Biden will be forced to take a realpolitik view that most of us will find hard to stomach. No matter how unjust Ukraine’s fate, he must continue to reject any measure that threatens to put U.S. troops in direct conflict with the Russians.

Anything else would be reckless. Yes, the absorption of Ukraine into Russia would be a human tragedy and geopolitical nightmare. But a shooting war between NATO and Russia would constitute an existential crisis that some large segment of the planet might not survive.

This might mean pressuring Zelensky to accept a negotiated solution that is patently unjust — if it’s even possible. No one wants to say it now, but America would sooner see Ukraine cede some territory than risk all-out war. It would be an imperfect solution, but one in which we’d all live to fight another day.