A Ukrainian soldier stands on a military vehicle at a checkpoint near the eastern Ukrainian city of Debalcevo, in Ukraine, 05 September 2014. The Ukrainian government and separatists signed a ceasefire agreement raising hopes for an end to months of fighting in the country's east. (Roman Pilipey/EPA)

PRESIDENT OBAMA and German Chancellor Angela Merkel have been insisting for months that there is no military solution to the crisis in Ukraine. Unfortunately, Russian President Vladi­mir Putin did not agree with them. When Ukraine’s army appeared on the verge of recapturing the last cities held by “rebel” forces marshalled and supplied by Russia, the Kremlin chief dispatched several thousand Russian regulars and hundreds of tanks and armored vehicles across the border, according to NATO officials. By Thursday, the Russian forces were on the outskirts of the key southern city of Mariupol, threatening to open a land corridor to the occupied province of Crimea.

Mr. Putin then presented Ukrainian President Petro Poroshenko with a “cease-fire” plan that required Kiev to capitulate to Russia’s continuing control over some 10 percent of Ukraine’s territory and even more of its population. With his defenses crumbling and no prospect of significant help from the West, Mr. Poroshenko had little choice but to agree. On Friday, he accepted an accord that could turn eastern Ukraine into another of the “frozen conflicts” Mr. Putin uses to destabilize and bully neighbors, including Georgia and Moldova.

That is, if the cease-fire holds. Western leaders were rightly suspicious that the “Putin plan” was little more than a ruse to forestall a new round of economic sanctions planned by the United States and European Union. Mr. Obama and British Prime Minister David Cameron took the correct approach by pushing for the adoption of the sanctions package and then waiting to see Mr. Putin’s next moves.

No one should be surprised if Russian forces renew their offensive in the coming days, while Moscow’s mendacious propaganda apparatus blames Ukraine. Even if Mr. Putin honors the agreement, Ukraine will start the follow-up negotiations in a deep hole. Mr. Poroshenko has previously demanded that Russia withdraw all its forces from Ukraine, that the border be handed back to Ukrainian government control and sealed and that the “rebel” forces disarm. In exchange, he has offered a sensible and generous package of political concessions, including decentralization of power to Russian-speaking areas.

If Moldova and Georgia are any guide, Russia will not withdraw its troops — which it still denies are in Ukraine. Moscow will seek to turn the eastern provinces into autonomous entities with a veto over Ukraine’s foreign policies. It will foster constant tension to ensure that Ukraine’s political system remains weak and unstable and its economy in ruins.

That prospect is what caused Mr. Poroshenko to launch the Ukrainian army’s attempt to retake the occupied territories. Kiev’s forces were successful in July and August, recapturing two-thirds of the area. If Mr. Putin had been deterred by Western governments from escalating to a direct invasion by Russian army forces, Ukraine’s fight to save itself could have succeeded. He was not.

At a news conference at the NATO summit in Wales on Friday, Mr. Obama claimed that Western sanctions had made the cease-fire possible. That’s one way of looking at it. Another is that the weak U.S. and European response to Russia’s aggression has placed Mr. Putin in a position to carve another piece from Ukraine.