The most important week for European security since the end of the Cold War ended the way it began—badly. A Russian invasion force pressed its gains in Ukraine, while the U.S. and its European allies mustered few meaningful actions to stop Vladimir Putin's assault on the international order.

At its biennial summit in Wales, the North Atlantic Treaty Organization—once known as the most successful and powerful military bloc in human history—missed opportunities large and small. The allies indicated they'd broaden sanctions on Moscow but failed to announce any.

In response to the Russian threat, NATO on Friday agreed to a "continuous" rotational presence of an unspecified number of troops in eastern Europe. It also approved a new "spearhead" rapid-reaction force headquartered in Poland, though the 4,000 troops pledged to it will be based elsewhere.

This raises the question of just how rapid the reaction would be if Mr. Putin decides to stir up trouble in, say, Estonia. As it happens, Estonia said on Friday that Russian troops had abducted one of its security officers at gunpoint in southeastern Estonia. Russia claimed he was in Russia.

NATO would have done better to move the thousands of American troops sitting idly at German bases forward to Poland and the Baltic states. This would have sent a clearer message to Moscow of NATO's seriousness by creating a tripwire against a Russian attack.

None of this will help the one European democracy currently fighting for its survival. Ukraine's President Petro Poroshenko attended the Wales summit and got a dose of Western reality. President Obama again pledged "support of Ukraine's sovereignty, independence and territorial integrity and its right to defend itself," but he again did nothing.

He and the allies again demurred on Ukraine's urgent requests for antitank and antiaircraft weapons, intelligence and military training. Ukraine sent thousands of troops to NATO missions in the Balkans and Afghanistan, fought for its freedom in the streets, and deserves better than this abandonment.

The West's abdication explains Mr. Poroshenko's decision to change course and agree to a cease-fire with Russia in eastern Ukraine. There's no way to spin Friday's deal as anything but a victory for Mr. Putin and a setback for independent Ukraine. Two weeks ago Mr. Poroshenko refused to talk with Mr. Putin's separatist proxies, whose leaders and many of whose fighters are from Russia. Ukraine's army had clawed back territory and besieged rebel strongholds.

Mr. Putin had to decide whether to let his proxy army lose or risk Western rebukes by invading. He chose to pour in thousands of regular Russian troops. Heavy artillery pounded Ukraine's underarmed forces, while Russia opened a new front against Mariupol, a strategic port city on the main road to Russian-held Crimea. President Obama refused even to call this an invasion.

So Mr. Poroshenko chose to save Mariupol and stop the bleeding, at least until Mr. Putin orders the rebels back into action to finish the land bridge he wants connecting Russia with Crimea. Mr. Putin now says he believes in "statehood" for a "New Russia" stretching across much of eastern Ukraine, and he'll have to take more territory to get there.

The Ukrainians will try to regroup militarily, hoping for belated foreign help. Poland, the Baltic states and Canada have considered lethal aid. The mood is also changing in Washington, at least outside the White House. The Democratic Chairmen of the Senate Armed Services and the Foreign Relations Committees this week called on President Obama to provide Ukraine with weapons.

American guns and missiles in Ukrainian hands wouldn't defeat the Russian army. But they might strengthen Ukraine's diplomatic leverage and raise the risks for Mr. Putin. The Kremlin is lying about the presence of Russian troops on Ukrainian soil to confuse the West and for domestic reasons. Rising casualties and an open war against Ukraine aren't likely to be popular for long in Russia.

In Estonia this week, Mr. Obama gave one of his better speeches promising that an attack on a fellow NATO member is an attack on America. Having taken Mr. Obama's measure for six years, and after this week of tough talk but soft actions, Mr. Putin is unlikely to believe it. A week that was supposed to make Europe safer may have invited more aggression from Moscow.