IT HAS been five weeks since Russia assaulted Ukrainian Navy ships that were passing through international waters near the occupied Crimean Peninsula. Though the world’s attention has moved on, Russian forces have not: On the contrary, they are massed near Crimea’s border with eastern Ukraine, prompting warnings from the Ukrainian government as well as independent observers that the regime of Russian President Vladi­mir Putin may be planning a new attack on the country. Rather than await the Russians’ next move, the Trump administration should be working with allies to actively deter further aggression.

Since the Nov. 25 naval attack, in which Russia seized three Ukrainian vessels and captured 23 crew members, its ships have maintained a stranglehold on the Kerch Strait, which connects the Black Sea to the Sea of Azov, and strictly limited ship traffic to Ukrainian ports. That activity blatantly violates international law and a treaty under which Russia and Ukraine agreed to share the Kerch Strait and Sea of Azov. But apart from President Trump’s cancellation of a planned meeting with Mr. Putin shortly after the original incident, Moscow has suffered no significant consequences.

Now, the Ukrainians and independent observers are warning that Mr. Putin may be planning a ground offensive. The Washington-based Institute for the Study of War reported on Dec. 23 that Russian military convoys had been moving toward the border between Crimea and Ukrainian-held territory, and that fighter jets had been redeployed to Crimean airfields. Though the purpose of the military movements is unclear, the ISW said, “The data suggests that Putin is preparing to attack.”

The Russian ruler has motive to escalate his war against Ukraine, which has been simmering for several years. He is eager to undermine Ukrainian President Petro Poroshenko, who faces a reelection contest in the spring. Mr. Putin was infuriated by a recent move by the Ukrainian Orthodox Church to establish its independence from the Russian church. And he may believe that Mr. Trump, who lately has been loudly repudiating a U.S. role as “the policeman of the world,” is in no mood to defend a remote piece of Ukraine from Russian tanks.

It could well be that Mr. Putin can accomplish his aims simply by raising the level of tension and by showing he can strangle Ukrainian ports with impunity. But whatever his intentions, the United States and its NATO allies should be acting to punish Mr. Putin for his coast guard’s actions in the Kerch Strait and to deter an escalation. There are a number of steps that could be taken, ranging from dispatching warships to the Black Sea, with a stop in the Ukrainian port of Odessa, to stationing NATO or European Union observers in eastern Ukraine. There have, as yet, been no sanctions against Russia specifically linked to its seizure of the Kerch Strait; there should be.

Mr. Trump still aspires to friendly relations with Mr. Putin. That won’t be possible if the Russian ruler initiates more aggression against Ukraine or other neighbors. It is consequently in Mr. Trump’s interest to make clear to Moscow that any further attacks will trigger a strong response from the United States.

Read more: