A tough Western reaction might have stopped the Russian offensive after last weekend. Now it is far more dangerous. The bloodshed reported Sunday may be taken by the Kremlin as an excuse to order a more overt invasion by the tens of thousands of troops it has massed near the border. The Ukrainian government, for its part, has rightly taken the position that it cannot allow Russia to take over its cities by force of arms without fighting back.

The Obama administration elected not to adopt significant measures last week in part because it was awaiting what it described as a diplomatic opening — a four-way meeting this week of foreign ministers from the United States, Russia, Ukraine and the European Union. Yet there is almost no chance this gathering, if it takes place at all, can lead to an acceptable solution for Ukraine. Moscow is demanding that the country be chopped up into pieces and that areas under its influence be given a veto over Ukraine’s foreign policy. Given the weak response to its aggression, Moscow has no incentive to drop that scheme.

It may be too late to prevent war in eastern Ukraine. But the United States must quickly take the measures promised by Mr. Obama and Mr. Kerry, or lose what little credibility it retains on Ukraine. If Sunday’s combat continues, it should also reconsider Kiev’s request for non-lethal supplies and small arms for its forces. If Ukrainians are forced to fight for their country, they should be helped.

Image: Russian soldiers seize more public buildings in Eastern Ukraine (photo: Voice of America)