PRESIDENT TRUMP’S relations with, and intentions toward, the Russian regime of Vladi­mir Putin remain troublingly opaque. So it was a pleasant surprise on Tuesday when his ambassador to the United Nations, Nikki Haley, excoriated Moscow for blocking action against the Syrian regime of Bashar al-Assad for its illegal use of chemical weapons. Russia and China, which joined in vetoing a sanctions resolution, “made an outrageous and indefensible choice,” Ms. Haley said. “They turned away from defenseless men, women and children who died gasping for breath when Assad’s forces dropped their poisonous gas. They ignored the facts. They put their friends in the Assad regime ahead of our global security.”

Ms. Haley was only stating plain truths. An investigation ordered by the U.N. Security Council concluded months ago that Syrian government forces dropped chlorine-filled barrel bombs on rebel-controlled areas three times in 2014 and 2015 — a blatant violation of the Chemical Weapons Convention that the Assad regime joined in 2013. In reality, Damascus has used the horrific weapons considerably more often than that; Human Rights Watch found that it dropped them systematically on civilians in Aleppo last year. Yet Moscow and Beijing refused to accept the evidence and tried to put off a vote on sanctioning those found responsible for the attacks.

Ms. Haley, who lunched with Mr. Trump and Vice President Pence the day before the Security Council’s meeting, insisted on a vote so that the Putin and Xi Jinping regimes were forced to go on the record. Then she bluntly called them out. “It’s a sad day on the Security Council,” she said. “When members start making excuses for other member states killing their own people, the world is definitely a more dangerous place.”

The ambassador’s forceful diplomacy was useful for more than sending the message that the Trump administration will be, like all U.S. administrations before it, ready to oppose war crimes. We hope it also is meant to put Mr. Putin on notice that Mr. Trump’s stated willingness to join with him in fighting the Islamic State will not extend to tolerating gross human rights violations or propping up the blood-soaked Assad dictatorship. An alliance with Russia that abetted such actions would only discredit the United States, including with its major allies in the Middle East; the chief beneficiary would be Iran, which has made the Assad regime its puppet and which has the largest interest in sustaining it.

Mr. Putin, who has frequently hinted that he could be willing to dispense with Mr. Assad, has been trying to orchestrate a new Syrian peace process with Turkey that all but excludes the United States. Predictably, it is going nowhere — in part because, as The Post’s Liz Sly reports, players on both sides are waiting to see what stance the new U.S. administration adopts. This is leverage that Mr. Trump should use: If Mr. Putin wants his help to settle the Syrian conflict and protect Russia’s interests there, he should be obliged to split with Iran and abandon a regime that drops chlorine on women and children.