President Trump now faces the gravest test yet to his leadership.

Russia's latest aggression in the Sea of Azov is for Trump the analogy of what Russia's 2014 downing of a Malaysian Airlines jet over Ukraine was for former President Barack Obama. Obama failed that test catastrophically, refusing pleas by our Western allies to truly confront Putin over these murders. This showed Putin that he had open season to challenge American interests. In the final two years of his presidency, Russia's escalation in both Ukraine and Syria gave bloody testament to Obama's error.

Trump faces a similar test of resolve now that Putin's navy has fired upon Ukraine's.

The moral and political import of what occurred last weekend is clear. As a Ukrainian flotilla of three naval vessels attempted to transit the Sea of Azov and dock at a Ukrainian port in Mariupol, Russian warships fired on them. The Ukrainian vessels and 23 crew members were seized. Outrageously, Russia blames Ukraine for this aggression. Simply, Russia decided to attack Ukrainian vessels in Ukrainian waters for no justifiable reason.

Russia is trying to further subjugate Kiev's democratic authority to the Kremlin autocracy. For months, the Russian security apparatus has steadily escalated their harassment of Ukrainian vessels transiting the Sea of Azov.

Russia is shredding a basic principle of the American-led international order: that sovereign nations should control their own borders and be free to trade with the world.

Russia claims that it has unilateral right to decide which vessels get to enter these waters because since its seizure of Crimea, Putin controls the Kerch Strait that separates the Sea of Azov from the Black Sea. But Russia is wrong. It is the unanimous verdict of the European Union, the U.S., and the vast majority of democratic nations that Crimea is Ukraine, not Russia. For all these reasons, Trump must educate Putin on the error of his latest aggression.

First, Trump should plainly, loudly, and publicly clarify that the blame here is entirely Russia's. He should condemn Russia's action for what it is: an act of wanton aggression against a far less powerful democratic nation. Second, and preferably in a joint statement with America's European allies, Trump should make clear that he expects Russia to immediately release the Ukrainian sailors and vessels it has seized. In that demand, Trump should be clear that absent proactive steps by Putin to de-escalate this crisis, the U.S. will pursue more coercive measures of restraint.

Start with sanctions. Trump should work with America's European allies to build a new sanctions portfolio that punishes Russia for this latest hostility. So as to avoid Russian perceptions that the U.S. is bluffing, Trump should immediately dispatch Treasury Secretary Steven Mnuchin or Secretary of State Mike Pompeo to Brussels and London. If Russia changes course, the sanctions threat can be lifted.

If Moscow doubles down, the U.S. and EU should restrain Russian capital flows outside its borders. That action will put an immediate pinch on Putin's pocket and greatly upset his oligarch base. Europe ought to be all for such an effort: After all, French President Emmanuel Macron now spends most of his time calling for bolder European leadership in the cause of Europe's security.

Still, sanctions threats are not enough alone. Recognizing that states like China, Iran, and North Korea always watch American presidents for signs of strength or weakness at moments of consequence such as this one, Trump should pick up the phone to Defense Secretary Jim Mattis. If Russia does not back down within the next week, we suggest a visit to Ukraine's port in Odessa by a flotilla of U.S. guided-missile destroyers.

Odessa has special significance in that it is the originating port from which the Russian-detained Ukrainian crews embarked. Visiting Odessa would thus represent a signal of American strength calibrated to avoid a conflict of the kind that might be sparked were the U.S. Navy to attempt entry through the Kerch Strait. And to counter the almost inevitable Russian harassment antics of our warships, they should meet a fighter escort from the Harry S. Truman carrier strike group. Ironically, the Truman group is currently underway in the Mediterranean Sea to deter Russian aggression in Syria and against Europe.

At moments such as this one, where an adversary is overtly shredding the most central basis of American order, active presidential leadership is needed. Russian President Vladimir Putin must rediscover American resolve.