Russia over the weekend mounted its most serious aggression against Ukraine since President Trump took office, and America punted.

Trump’s outgoing ambassador to the United Nations, Nikki Haley, described the chain of events that led to the current standoff at the Black Sea as an “outrageous violation of sovereign Ukrainian territory.” She vowed that Washington and its friends would stand with Kiev.

So what’s the plan? “It is our expectation that our European partners will lead this effort through the Normandy Four format,” Haley said, referring to a toothless Barack Obama-era gabfest that has mostly served to legitimize Moscow’s illegal annexation of Crimea and invasion of eastern Ukraine.

Trusting that diplomatic channel is like showing up to a gunboat fight without even a knife.

Consider what happened: Three Ukrainian vessels headed from Ukraine’s Black Sea port of Odessa to a Ukrainian port in the Sea of Azov. In the Kerch Strait, which separates the two bodies of water, Russian warships fired at the Ukrainians, injured crew members and seized the vessels.

Russia and Ukraine are already at war. Since their naked land grabs in 2014, the Russians have regularly breached Ukraine’s territorial integrity in the east, where thousands of well-armed Russian troops and paramilitary forces are permanently stationed.

The current crisis, though, is different. Interrupting freedom of navigation threatens not only Ukraine but all of Europe — and beyond. A weak Western response will invite further Russian infringement of former satellite states’ territories.

Things could quickly get out of hand. The Russian public is souring a bit on President Putin. A persistently sluggish economy has somewhat dimmed the image of the new czar.

A little war on behalf of the motherland, then, may return some color to Mr. Putin’s cheeks. On Monday, Russia’s deputy ambassador to the United Nations, Dmitry Polyanskiy, claimed the entire Sea of Azov for Russia and gave notice that his country would “curb” any Ukrainian or American “provocation” there.

Meanwhile, Ukraine’s president, Petro Poroshenko, imposed 30 days of martial law, warning that Russia is preparing another invasion. The move effectively muzzles all political opposition to the president’s leadership. It’s possible Mr. Poroshenko hopes tensions will assure his victory in next March’s presidential election.

The domestic politics aside, for Washington this incident implicates an important strategic principle. America, commanding the world’s strongest navy, is the final guarantor of free navigation across the globe. The president can’t permit this weekend aggression to go unchecked.

It won’t do to rely on some lame European diplomatic protocol. Mr. Trump can’t hope to lead from behind France and Germany, both weak on Russia. That’s not how to send a serious message to Putin.

“The United States will maintain its Crimea-related sanctions against Russia,” Mrs. Haley promised Monday. (Maintain? That’s it?) She added that Russia’s standing in the world will be undermined and further escalation will sour its relations with America and others.

Russia hawks inside Mr. Trump’s inner circle must cringe at such Obama-like rhetoric. Mrs. Haley is one of those hawks, and at least she forcefully condemns Russia. Not so the White House, which kept mum Monday. Perhaps it’s planning biting new sanctions, canceling all future Putin meetings with Mr. Trump, or arming Kiev with more defensive weapons.

Other Trump advisers are sure to urge a return to the policies of George W. Bush and Barrack Obama in the aftermath of Russia’s aggressions against Georgia and Ukraine, respectively. The formula: Make speeches at the United Nations, threaten “loss of legitimacy,” impose mild sanctions. Rinse. Repeat.

Meanwhile, the Kremlin becomes bolder.

Mr. Trump started out by sending lethal arms to Ukraine, which Mr. Obama had refused to do. Washington has done little since, though, even as Mr. Trump has struggled to decisively counter critics who caricature him as Mr. Putin’s marionette.

The latest Ukraine crisis gives Mr. Trump a chance to show he can stand up to the Russian bully. His hesitant response shows he may not be up to the challenge.

This column first appeared in the New York Post.