WASHINGTON — “The most significant reinforcement of our collective defense any time since the Cold War,” President Obama called it. A bit of an exaggeration, perhaps, but it was still an achievement: Last week’s NATO summit in Warsaw ordered the deployment of troops to Eastern Europe, the alliance’s most serious response yet to Russia’s aggression and provocations on its western frontier.

The post-Ukraine economic sanctions have been weak; the declamatory denunciations, a mere embarrassment. They’ve only encouraged further reckless Russian behavior — the buzzing of U.S. ships, intrusions into European waters, threats to the Baltic States.

NATO will now deploy four battalions to front-line states. In Estonia, they will be led by Britain; in Lithuania, by Germany; in Latvia, by Canada; in Poland, by the United States. Not nearly enough, and not permanently based, but nonetheless significant.

In the unlikely event of a Russian invasion of any of those territories, these troops are to act as a tripwire, triggering a full-scale war with NATO. It’s the kind of coldblooded deterrent that kept the peace in Europe during the Cold War and keeps it now along the DMZ in Korea.

In the more likely event of a “little green men” takeover attempt in, say, Estonia (about 25 percent ethnically Russian), the sort of disguised slow-motion invasion that Vladimir Putin pulled off in Crimea, the NATO deployments might be enough to thwart the aggression and call in reinforcements.

The message to Putin is clear: Yes, you’ve taken parts of Georgia and Ukraine. But they’re not NATO. That territory is sacred — or so we say.

This is a welcome development for the Balts, who are wondering whether they really did achieve irreversible independence when the West won the Cold War. Their apprehension is grounded in NATO’s flaccid response to Putin’s aggressive revanchism, particularly in Ukraine. Obama still won’t provide Ukraine with even defensive weaponry. This follows years of American accommodation of Putin, from canceling a Polish-Czech missile defense system to, most recently, openly acquiescing to Russia’s seizure of a dominant role in Syria.

And what are the East Europeans to think when they hear the presumptive presidential candidate of the party of Reagan speaking dismissively of NATO and suggesting a possible American exit?

The NATO action takes on even greater significance because of the timing, coming just two weeks after Brexit. Britain’s withdrawal threatens the future of the other major pillar of Western integration and solidarity, the European Union. NATO shows that it is holding fast and that the vital instrument of Western cohesion and joint action will henceforth be almost entirely trans-Atlantic — meaning, under American leadership.

The EU, even if it doesn’t dissolve, will now inevitably turn inward as it spends years working out its new communal arrangements with and without Britain. Putin was Brexit’s big winner. Any fracturing of the Western alliance presents opportunities to play one member against another. He can only be disappointed to see NATO step up and step in.

After the humiliating collapse of President Obama’s cherished Russian “reset,” instilling backbone in NATO and resisting Putin are significant strategic achievements. It leaves a marker for Obama’s successor, reassures the East Europeans and will make Putin think twice about repeating Ukraine in the Baltics.

However, the Western order remains challenged by the other two members of the troika of authoritarian expansionists: China and Iran. Their provocations proceed unabated. Indeed, the next test for the United States is China’s furious denunciation of the decision handed down Tuesday by the Permanent Court of Arbitration in The Hague — a blistering, sweeping and unanimous rejection of China’s territorial claims and military buildup in the South China Sea.

Without American action, however, The Hague’s verdict is a dead letter. Lecturing other great powers about adherence to “international norms” is fine. But the Pacific Rim nations are anxious to see whether we will actually do something.

Regarding Iran, we certainly won’t. Our abject appeasement continues, from ignoring Tehran’s serial violations of the nuclear agreement (the latest: intensified efforts to obtain illegal nuclear technology in Germany) to the administration acting as a kind of Chamber of Commerce to facilitate the sale of about 100 Boeing jetliners to a regime that routinely uses civilian aircraft for military transport (particularly in Syria).

The troop deployments to Eastern Europe are a good first step in pushing back against the rising revisionist powers. But a first step, however welcome, seven and a half years into a presidency, is a melancholy reminder of what might have been.

Charles Krauthammer’s email address is letters@charleskrauthammer.com.

(c) 2016, The Washington Post Writers Group