AS RUSSIAN soldiers and tanks advance across southeastern Ukraine, the Obama administration and the NATO alliance are making a show of defending several nations that lie far away and that are not presently under attack. President Obama is due Wednesday in the Baltic republic of Estonia, and a NATO summit on Thursday and Friday is expected to approve several measures, including a new rapid-reaction force, to bolster the defenses of Estonia and other Eastern European nations that have joined NATO in the past 15 years.

The defensive measures are justified and, if anything, inadequate: NATO still shrinks from scrapping a 17-year-old agreement with Russia not to establish permanent bases in Central and Eastern Europe even though Moscow is flagrantly violating its own European commitments. But the new force will do nothing to stop what Ukrainian President Petro Poroshenko rightly calls the “direct, unconcealed aggression” by Russia against Ukraine or the mounting ambitions of Russian President Vladi­mir Putin, who is calling the area under attack by the 19th-century tsarist name “Novorossiya” and suggesting that its “statehood” should be under negotiation.

Intentionally or not, the White House and NATO are sending Mr. Putin the message that Ukraine can be sacrificed. “Ukraine is not a member of NATO,” Mr. Obama gratuitously stated last week while downplaying the Russian invasion as nothing new. A White House official told reporters that the president’s message to Moscow is, “Don’t even think about messing around in Estonia or in any of the Baltic areas in the same way that you’ve been messing around in Ukraine.”

But what about the “messing around” in Ukraine? Mr. Poroshenko has been invited to the NATO summit, and there is discussion of providing some funding for Ukraine’s defense or help with intelligence and logistics. That won’t be nearly enough to stop the Russian forces from overrunning more territory. Ukraine will soon be confronted with the terrible choice of fighting alone against the Russian invasion or capitulating to Mr. Putin’s demands.

Mr. Obama has publicly ruled out — again, with gratuitous clarity — using force to defend Ukraine, and some European members of NATO balk at supplying lethal aid. But Russia’s invasion could still be checked, or at least slowed, by concerted action this week. Individual NATO members, including the United States, should immediately begin supplying the Ukrainian army with lethal weapons, as congressional leaders from both parties have been urging. Ukraine needs anti-tank and anti-aircraft weapons, drones, spare parts and fuel, among other things, and these are readily available in NATO members’ arsenals.

As importantly, sanctions against the Russian economy can be significantly escalated. The good news is that U.S. and European officials have worked out a relatively strong package of measures, and the European Union has committed itself to act by the end of the week. But it’s not clear that the stringent steps that might give Mr. Putin pause will ultimately win the necessary support of all 28 E.U. members. If they do not, the Obama administration should prepared to quickly adopt those that can be effective without European backup.

Russia’s aggression in Ukraine poses a critical test to the Western alliance, and the war there is at a tipping point. The response cannot be to cede Ukraine while trying to dissuade Mr. Putin from further conquests.