A displaced Syrian girl who was wounded by shelling sits at a camp in Kafr Lusin near the Bab al-Hawa border crossing with Turkey in the northern part of Syria’s rebel-held Idlib province on Thursday. (Aaref Watad/AFP/Getty Images)

CAN A tweet stop another bloodbath in Syria? Evidently not. Hours after President Trump used his favorite medium to warn Russia not to join in a new offensive on the northern province of Idlib, saying it would be “a grave humanitarian mistake,” Russian aircraft carried out dozens of strikes in the rebel-held province on Tuesday, reportedly killing at least a dozen civilians. That was just the beginning of what could be the most horrific episode yet in the seven-year-old civil war. About 3 million civilians are trapped in Idlib, of whom as many as half are refugees from other parts of Syria. Mr. Trump was hardly exaggerating when he wrote that “hundreds of thousands of people could be killed.”

As Secretary of State Mike Pompeo noted, Russia pledged to preserve Idlib as a “deescalation zone” and end the war through diplomacy. Yet it now appears poised to throw its planes behind an assault by the government of Bashar al-Assad, also backed by Iran, to capture the territory by force. Moscow cites the presence of an al-Qaeda-linked rebel group, which controls much of Idlib; Foreign Minister Sergei Lavrov says it is a “festering abscess” that must be “liquidated.” But the United Nations’ Syria envoy estimates there are only about 10,000 fighters in the extremist group. If they follow their usual modus operandi, the Assad, Iranian and Russian forces will fight them by systematically targeting the civilian population and infrastructure, including food markets, schools and hospitals.

The damage could spread far beyond Syria. A full-scale offensive could send hundreds of thousands of refugees streaming north toward Turkey, where some 3.5 million Syrians already are harbored. For now, Turkish President Recep Tayyip Erdogan is preventing refugees from continuing on to Europe, where their mass arrival in 2015 prompted a far-reaching political backlash. But if Idlib erupts, that could change.

Faced with these stakes, the Trump administration’s response has been pathetically weak. Mr. Pompeo resorted to tweeting that “the world is watching” — echoing the feckless words used by then-Secretary of State John F. Kerry when Syrian and Russian forces reduced the city of Aleppo to rubble. The only red line drawn by the White House is against the use of chemical weapons; in that case, a statement said, it would “respond swiftly and appropriately.” Deterring chemical attacks is important, but to stand by while a humanitarian catastrophe is created by conventional means, and the Assad-Russia-Iran alliance consolidates control over Syria, would be another damaging abdication of U.S. leadership.

At this point, the slim hopes for averting mass bloodshed appear to rest with Turkey, a longtime supporter of Idlib rebel forces. Mr. Erdogan is expected to participate in a three-way summit with Russia and Iran on Friday. Mr. Erdogan could seek to broker a solution that would aim to isolate and neutralize the al-Qaeda force with the help of other Syrian fighters, rather than a general assault on the province. Unfortunately, the Assad regime and its Russian allies have a long record of rejecting compromise for scorched earth. They will also judge that tweets from the president of the United States can be safely disregarded.