Ukrainian President Petro Poroshenko speaks to lawmakers in parliament in Kiev, Ukraine, on Nov. 26. (Stepan Franko/EPA-EFE/Shutterstock)

RUSSIA’S NEW aggression against Ukraine demonstrates, among other things, how little Vladi­mir Putin worries about pushback from a divided Europe or a U.S. government led by President Trump. On Sunday, Russian forces assaulted three Ukrainian navy ships that were attempting to pass through the Kerch Strait near the Russian-occupied Crimean Peninsula, ramming one of them and opening fire on the others. The Ukrainian boats were seized along with 23 crew members, a number of whom were wounded.

This blatant act of war, which clearly violated both international law and a treaty between Ukraine and Russia, culminated a creeping campaign by Russia to blockade Ukrainian ports on the Sea of Azov, near Crimea. Shipping from those cities must pass through the Kerch Strait to reach the Black Sea and the Mediterranean. Following the naval attack, Ukrainian President Petro Poroshenko warned of a possible Russian ground offensive, but Mr. Putin has already advanced key objectives, including boosting his own flagging popularity at home, destabilizing Ukraine ahead of a presidential election and testing the reaction of Western leaders, whom he is due to meet at a Group of 20 summit later this week.

To be sure, most Western governments sided with Ukraine, and some reacted strongly: British Foreign Secretary Jeremy Hunt condemned what he called “Russian contempt for international norms and Ukrainian sovereignty.” But Mr. Putin could be only pleased by the response from Mr. Trump, who since Sunday has criticized European allies, Mexico, Robert S. Mueller III, “60 Minutes” and CNN but offered no direct rebuke of Russia. Instead, when asked about the incident Monday, he said, “We do not like what’s happening, either way,” suggesting that Ukraine and Russia were equally at fault. The strongest administration statement came from outgoing U.S. Ambassador to the United Nations Nikki Haley, who called the Russian actions “outrageous” and “outlaw actions” but essentially ruled out any tangible U.S. response.

That is a far-from-adequate response. The United States should be moving firmly to deter further aggression by Russia and to restore Ukraine’s right to send ships through the Sea of Azov and the Kerch Strait. Russian ports in the region could be threatened with sanctions. Ukraine could be supplied with anti-ship missiles that would allow it to better defend itself.

At a minimum, Mr. Putin’s aggression ought to affect the reception he receives from Mr. Trump at the G-20 summit in Buenos Aires. The Kremlin said Monday that a meeting between the two leaders was “being prepared” regardless of the naval attack, though the White House did not confirm that. Mr. Trump could send a message by refusing to meet Mr. Putin; if he does go ahead with a bilateral meeting, he ought to begin it by telling the Russian leader that his assault on Ukraine is unacceptable and will have consequences. A failure to do so would simply confirm Mr. Putin’s evident belief that further aggression against Ukraine will cost him nothing with this U.S. president.