AS HEAVY fighting resumed Tuesday in eastern Ukraine , it was obvious that Russian President Vladi­mir Putin had disregarded the demands from the United States and European Union that Russia stop intervening. Rebels backed by Moscow did not hand back border posts; military supplies have not stopped flowing across the border; Mr. Putin did not compel the insurgents to observe a cease-fire, leaving the Ukrainian government with no choice but to resume military operations. As NATO commander Philip Breedlove put it, “what we see is continued conflict [and] continued support of the conflict from the east side of the border.”

That means that President Obama and E.U. leaders face the test of delivering on their rhetoric. Mr. Obama said on June 5 that if Mr. Putin “over the next two, three, four weeks . . . remains on the current course,” new sanctions would be applied to the Russian economy. Four weeks have passed. Last Thursday Secretary of State John F. Kerry said that “it is critical for Russia to show in the next hours, literally, that they are moving to help disarm the separatists.” Many hours have passed with no disarmament. On Friday, E.U. leaders set a Monday deadline for a series of steps, including the evacuation of border posts. These have not been carried out.

A failure by the West to act following such explicit rhetoric would be a craven surrender that would provoke only more Russian aggression. Yet so far the signs are not good. Despite promising to respond “without delay” if its ultimatum was not heeded, E.U. leaders on Tuesday reportedly postponed any response until next week. The Obama administration, for its part, is hiding behind the Europeans, saying it cannot move unless and until they do. That makes U.S. credibility contingent on accord among 28 nations, including Cyprus, Greece and several others friendly to Russia.

Businessmen on both sides of the Atlantic warn about the potential cost of measures aimed at the Russian energy or banking industries. But the three sanctions under discussion are modest: Officials call them “surgical.” They could restrict future investment in new Russian energy projects or limit technology transfers. They will neither inflict serious damage on the Russian economy nor cost Western jobs. Yet they are needed to restore pressure on the Russian stock market and ruble, both of which have rebounded in recent weeks in the absence of Western action.

Administration officials say they still have hope of reaching agreement with Europe on a sanctions package, and certainly joint U.S.-E.U. action would be preferable. Mr. Putin would like nothing better than to drive a wedge between the allies over Ukraine. But the worst outcome would be inaction, even as Ukraine’s army desperately attempts to contain a hardened insurgent force, including many Russians, that has been equipped and advised by Moscow. The relative good news in recent weeks has been Mr. Putin’s tempering of his aggression out of fear of sanctions. If there is no response, he will lose that fear.

A pro-Russian militant looks through the scope of his rifle during the storm of the regional police station in the centre of the eastern Ukrainian city of Donetsk. (Alexander Khudoteply/AFP/Getty Images)