Hints from the Kremlin suggest that this is how Mr. Putin hopes to gain lasting ground from his botched invasion. Mr. Putin insists that Ukraine accept the permanent loss of the three pieces of the country that Russia already has taken—the Crimean Peninsula and the provinces of Donetsk and Luhansk.

That might sound like a clean-cut outcome, but it wouldn’t be. If Russia wins the terms it demands, then other parts of Ukraine will fall under the shadow of contrived secessions, sham independence movements and whatever other forms of subversion Mr. Putin might improvise. Brutal client outposts, fake referendums and forced population exchanges likely won’t be limited to two or three parts of Ukraine. Even if Mr. Putin makes a show of restraint at first, he would have convenient launchpads from which to do more harm. Under the endgame he likely is pursuing, Ukraine can expect no better a fate than Georgia, Moldova or Azerbaijan, which endured coercion by Russia for decades.

The West tolerated Russia’s strategy of creating and sustaining “frozen conflicts” against its smaller neighbors. This signaled to Mr. Putin in 2014 that it was safe to seize Crimea and conjure the rebel groups through which Russia has ruled Donetsk and Luhansk and menaced Ukraine. Tolerating such an outcome in Ukraine would be a mistake.