RUSSIA AGAIN appeared on the verge of invading Ukraine over the weekend, this time in the guise of a “humanitarian operation.” President Obama and other Western leaders sounded the alarm, warning that the prospective intervention “is unacceptable, violates international law and will provoke additional consequences,” as a White House statement put it. For his part, Ukrainian President Petro Poroshenko agreed to a non-military relief operation under the auspices of the Red Cross that would allow for Russia’s participation.

Whether that would be enough to deter Russian ruler Vladi­mir Putin wasn’t clear on Monday. According to NATO Secretary General Anders Fogh Rasmussen, tens of thousands of Russian troops remained poised on Ukraine’s border; he said “there is a high probability” of invasion. Though a vacationing President Obama is already overseeing U.S. air strikes in Iraq, the United States and its allies must be prepared to act quickly if Russian military forces cross the frontier.

The motive for another escalation in Russia’s ongoing meddling is clear enough: not the “humanitarian crisis” the Kremlin claims is occurring in areas held by its surrogate forces but the threat that the Ukrainian army and allied militias will win a military victory. Government spokesmen say Kiev’s forces have succeeded in surrounding the cities of Donetsk and Luhansk, where the remaining Russian-backed forces are concentrated, after recapturing three-quarters of the territory they held. So Mr. Putin faces the collapse of his proxy force, a development that would not only loosen his hold on Ukraine but also potentially lead to political trouble at home — where state propaganda has whipped up a nationalist fervor over Ukraine.

To their credit, Western leaders who once pressed Mr. Poroshenko to accept a cease-fire deal tilted toward Russia have not tried very hard to stop the Ukrainian military offensive. In a phone call Monday, Mr. Obama urged Mr. Poroshenko “to continue to exercise restraint and caution in military operations in order to avoid civilian casualties,” according to a White House statement, but did not say the operations should stop. The continued fighting risks providing Mr. Putin with a pretext for “humanitarian” intervention. Western leaders, though, appear to accept Mr. Poroshenko’s argument that the military operation is not about defeating Russia but saving Ukraine. If Mr. Putin’s forces are able to hold onto a piece of territory, the Russian president will be able to block Ukraine’s stabilization indefinitely, just as he has used “frozen conflicts” to sabotage other Russian neighbors.

Mr. Poroshenko is still offering a peace plan that involves a cease-fire and political dialogue on the condition that Ukraine’s border is sealed to further infiltrations of Russian weapons and fighters. That could perhaps provide a face-saving exit for Mr. Putin, but it’s one Moscow is unlikely to embrace unless its proxy forces are on the verge of defeat. That’s why the Ukrainian military operation should continue with Western support, including fresh aid for the army, and why the United States and its allies should do everything possible to deter Mr. Putin’s “humanitarian” invasion. What “additional consequences” can Moscow expect if it crosses the line? A robust package should be readied and telegraphed to the Kremlin.