In 1991 U.S. forces easily hurled the Iraqi Army out of Kuwait. That option isn’t even considered today because Russia has nuclear weapons. A disturbing lesson is that having nuclear weapons enhances an aggressor’s inclination to attack with conventional forces. In any putative assault against Taiwan, Xi Jinping is likely to threaten the use of nuclear weapons to deter the U.S. from intervening with conventional force. For Mr. Putin to emerge intact after tearing apart Ukraine sets a terrible precedent.

If Russian troops withdraw from Ukraine but Mr. Putin is still in charge in Moscow, it will be a severe defeat for America. In his meeting with all 30 NATO nations, Mr. Biden must cross his Rubicon. He must declare that the sanctions crippling Russia will remain in full force, with no exit ramps, as long as Mr. Putin remains in power. America’s objective isn’t a return to the status quo ante; it requires removing Mr. Putin in order to reinforce, or perhaps restore, America’s role as the defender of liberty. If Mr. Biden doesn’t clearly state that objective at this week’s NATO meeting, when the world is united behind him, he won’t do so in the future.