PERHAPS IT is just a coincidence that the worst chemical weapons attack in Syria since 2013 came only a few days after the Trump administration confirmed that it would not seek to remove blood-drenched dictator Bashar al-Assad from power. Like Sen. John McCain (R-Ariz.), we suspect not. Either way, the horrific assault Tuesday on a rebel-held town will test whether President Trump will tolerate flagrant crimes against humanity by the Assad regime. So far, the signs are not good.

Though not all the facts are in, the early evidence reported from the scene was ominous. In an early-morning raid, witnesses said, Syrian planes bombed the community of Khan Sheikhoun, in northern Syria, with chemical agents that, according to posted videos, caused victims to foam at the mouth and struggle for breath. Syrian sources reported that at least 58 people were killed, including 11 children, and hundreds of others affected. Some died when a second air raid targeted one of the clinics where people were being treated.

United Nations investigations have established that the Assad regime has dropped barrel bombs filled with chlorine gas on civilians on multiple occasions since agreeing in 2013 to hand over its chemical arsenal and abide by a treaty banning chemical-weapons use. The Tuesday attack appeared even more serious: Medical personnel on the scene cited symptoms consistent with exposure to nerve agents, such as sarin.

It was a sarin attack near Damascus in August 2013 that prompted President Barack Obama first to propose, and then to retreat from, punitive military action against the Assad regime. Mr. Obama later described himself as “very proud” of his decision, because it led to a deal that supposedly eliminated the Syrian chemical stockpile. Tuesday’s attack underlined that Mr. Obama failed to accomplish even that goal, while his withdrawal from the scene opened the way to the destruction of the moderate Syrian opposition, the growth of the Islamic State and the intervention in Syria by Russia.

Now it is Mr. Trump’s turn to decide whether to stand up to Mr. Assad and his Iranian and Russian sponsors. So far he is ducking: A statement issued in his name said the attack was “reprehensible” and “cannot be ignored by the civilized world,” but then quickly pivoted to blaming the Obama administration for its “weakness and irresolution.” Meanwhile, appearing irresolute, Secretary of State Rex Tillerson declined to respond to a question about the attack before a meeting with Jordan’s King Abdullah II; later he issued a statement weakly calling on Russia and Iran to hold the Assad regime accountable.

To its credit, the new administration excoriated Russia and China on Feb. 28 when they blocked a U.N. Security Council resolution sanctioning Syria for its documented use of chlorine. The two governments, charged U.S. Ambassador Nikki Haley, “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.”

Will Mr. Trump now do the same?