On November 25, Russian military forces opened fire on three Ukrainian ships off the coast of Crimea, rammed one of them, and seized all three. The ships were manned by 23 crew members. Ukrainian authorities say between three and six were injured.

Russia claims the boats had illegally entered its sovereign waters, but this is untrue. The Ukrainian vessels—two gunboats and a tugboat—were sailing from Odessa around the Crimean peninsula in the Black Sea toward the Kerch Strait and Mariupol on the north bank of the Sea of Azov. A 2003 treaty between Ukraine and Russia guarantees both nations the right to use the Kerch Strait and Azov Sea for commercial purposes. The same treaty allows both nations to use the waters to transport military vessels so long as the transporting nation notifies the other. A Russian oil tanker nonetheless blocked the ships from passing through the strait, fighter jets passed overhead as if the three were an invading force, and Russian troops boarded and took control of the ships.

Ukraine insists that it notified Russia of the tiny flotilla, and there is no reason to doubt its statement. Russia claims, in typically melodramatic fashion, that the ships “crossed the Russian state border and illegally entered the temporarily closed waters of the Russian territorial sea.”

This stunt isn’t about protecting Russia’s borders from aggression. It is a test. Vladimir Putin wants to know how far the United States is willing to go to check Russian expansionism.

The testing began in earnest in March 2014, during the previous administration, when Moscow annexed the Crimean peninsula under the pretense of protecting a pro-Russian minority. (Russia now considers Crimea part of Russia, although Ukraine and most of the rest of the world hold it to be an illegal annexation.) At the same time, Russian-backed mercenaries streamed into eastern Ukraine to back pro-Russian separatists for the purpose of taking eastern Ukraine for Russia. More than 10,000 have died in the conflict. The fighting goes on despite the Minsk II cease-fire signed in February 2015.

Ukraine has responded to this quiet invasion by vastly increasing its military preparedness. The East European nation has doubled the size of its army in just four years: It now has around 250,000 active-duty soldiers and roughly 80,000 reservists. They are also better equipped. In 2017, the Trump administration approved the sale of lethal defensive weapons to Ukraine.

In short: At any moment Crimea and eastern Ukraine could explode into a large-scale hot war between proxies of the United States and Russia. Vladimir Putin wants to consolidate his gains in Ukraine—wants to make them de jure instead of merely de facto, as they are now—and has every reason to precipitate low-level crises in order to find out how feasible this is. President Trump has only encouraged such testing by fawning on the Russian dictator at every opportunity, even to the point of publicly taking Putin’s word over that of his own national security advisers.

Trump likes authoritarians who like him—or at least pretend to. In recent months, he’s had kind words for China’s Xi Jinping, the Philippines’ Rodrigo Duterte, North Korea’s Kim Jong-un, and Saudi Arabia’s Mohammed bin Salman. Trump publicly contradicted the assessment of the U.S. intelligence community that MbS directed the murder of Jamal Khashoggi and made clear that whatever punishment follows the brazen murder of the prominent Saudi regime critic, the Saudi monarchy will escape serious consequences. If Putin had concerns that the president might take a real stand against new Russian aggression, Trump’s figurative shrug of the shoulder over the Saudi crime had to ease them.

How will the United States respond to the Black Sea incident? It’s anybody’s guess. U.N. ambassador Nikki Haley issued a robust criticism of Russia’s aggression. But she’s leaving the administration. Secretary of State Mike Pompeo’s condemnation included forceful language but lacked any specific promise of consequences. As Russian dissident Garry Kasparov put it on Twitter: “Translate this US statement on Russia’s latest act of war against Ukraine into dictator-speak, Putin’s language: ‘We aren’t going to do anything about it.’ That’s how he will read it. Putin will scan this looking for ‘unless Russia . . .’ or ‘if Russia doesn’t comply . . .’ and, seeing nothing like that, no deterrence, he will continue as planned.”

Trump’s own comments gave Putin no reason to take a different view: “We do not like what’s happening either way,” Trump said while leaving the White House on November 26. “And hopefully it will get straightened out.”

Either way? Hopefully it will get straightened out? In response to widespread international condemnation of the Black Sea incident, Putin upped the ante: restricting maritime traffic into the Sea of Azov (which a Kremlin spokesman blamed on “bad weather”), beefing up the Russian military presence on Ukraine’s border, and deploying more S-400 missile systems to Crimea. Moscow also aired confessions from three sailors that were obviously forced.

On November 29, Trump sensibly canceled his summit with Putin set for the G20 meetings in Argentina. But getting this “straightened out” will require much more than putting off a meeting. It will require insisting Russia immediately free the vessels and release the crews. It will require Ukraine to have full rights to traverse the seas around its borders and a clear message that failure to afford Ukraine these rights will result in diplomatic and military consequences. More than anything else, it will require the president to lead rather than follow.