Local men ride bicycles in front of a damaged building in Uglegorsk, Donetsk area, Ukraine, 24 February 2015. (Luca Piergiovanni/EPA)

WHEN UKRAINE’S president, pressed by the leaders of France and Germany, signed a peace agreement for eastern Ukraine on Feb. 12, he hoped it would purchase several months of calm during which the beleaguered government in Kiev could move ahead with economic reforms and bolster its military defenses. That didn’t happen: Russian forces launched a major attack immediately after the conclusion of the agreement and captured a key town, Debaltseve. That raised the question of whether Western leaders would take action to punish Russian President Vladi­mir Putin’s violation and impede further aggression.

So far, they haven’t. While senior U.S. and European officials are threatening more economic sanctions against Russia, they are suggesting that they will act only if there is still more large-scale aggression by Moscow. “If this failure continues . . . there will be further consequences,” Secretary of State John F. Kerry said Saturday. On Tuesday, British Prime Minister David Cameron said there would be sanctions “if there is another Debaltseve.” For Debaltseve itself, it appears, Mr. Putin will pay no price.

That limp response is one reason Ukrainian leaders are convinced that Russia will soon renew its aggression. Andriy Parubiy, first deputy chairman of parliament, told us Tuesday in Washington that Mr. Putin’s next objective will likely be Mariupol, a port that would help to consolidate the puppet state he is constructing in eastern Ukraine and open the way to connecting it with Russian-occupied Crimea. Mikheil Saakashvili, the former Georgian president now working as an adviser to Ukrainian President Petro Poroshenko, said he expects Mr. Putin will seek to seize all of southern Ukraine, connecting his territories to the Russian-occupied Moldovan province of Transnistria and severing Ukraine’s access to the Black Sea.

Mr. Parubiy said the assault on Debaltseve showed that Mr. Putin’s support for occasional cease-fire agreements in Ukraine is merely tactical. By agreeing to the latest one, he was able to break what looked like political momentum in Washington toward providing Ukraine with desperately needed defensive weapons. President Obama said Feb. 9 that he was considering arms supplies, but the cease-fire three days later gave him reason not to act. The gross violation at Debaltseve and continued clashes near Mariupol since then have stirred no reaction from the White House.

Though senior officials at the State and Defense departments favor arms supplies for Ukraine, Mr. Obama appears sympathetic to German Chancellor Angela Merkel’s claim that they would provoke further escalation. Mr. Parubiy convincingly argues otherwise. Mr. Putin, he says, already has deployed every major Russian weapons system to Ukraine other than aircraft and nuclear weapons, so there is little room for further escalation. If Ukrainian forces were able to track Russian tanks with drones and strike them with antitank missiles, an assault on Mariupol would be costly for Mr. Putin and might be turned back.

As it is, the failure of the West to provide military support “is an invitation to Putin to proceed,” Mr. Parubiy says. “And he will proceed, until he is stopped by force.” By withholding military aid from Ukraine and declining to punish the Debaltseve attack, Mr. Obama is risking the destruction of the Kiev government he says he supports — and inviting a larger conflict with Russia.