This photo released on Monday, Sept. 10, 2018 by the Syrian Civil Defense group known as the White Helmets, shows smoke rising from a Syrian government airstrike, in Hobeit village, near Idlib, Syria. High-level diplomats from Iran, Russia and Turkey were meeting Tuesday with the U.N. envoy for Syria about creating a committee to revise the war-battered country's constitution.

A showdown is looming in Syria between Russia and the United States as pressure grows on President Donald Trump to retaliate if Russia and Syria use undue force in Idlib province. That’s an extremely dangerous position, but the threat of U.S. action should be part of a broader, urgent diplomatic effort to halt the assault before it becomes a catastrophe for civilians.

Russia, the Syrian government of Bashar al-Assad and Iran have rejected last-minute humanitarian pleas from Turkey and President Trump, and stepped up their bombing campaign against militants in Idlib, including the reported use of indiscriminate barrel bombs against civilian targets.

An estimated 2 million civilians are in danger of death, injury or displacement. The United Nations is warning of one of the worst humanitarian catastrophes of the 21st century.

The White House has announced that it has evidence Syria is preparing to use chlorine gas and perhaps other chemical weapons, adding to the sense of urgency.

Mr. Trump has rightly drawn a red line against the use of chemical weapons and has twice ordered limited bombings in Syria in response to such attacks. But Turkey’s President Recep Tayyip Erdogan, in an op-ed published Monday by the Wall Street Journal titled “The World Must Stop Assad,” also rightly observes that Syria uses other equally deadly techniques.

Saying that the threat of U.S. military action is needed, among other diplomatic steps, to dissuade Assad from launching a murderous attack on Idlib, Mr. Erdogan wrote, “It is crucial for the U.S., which has concentrated on chemical attacks, to reject its arbitrary hierarchy of death. Conventional weapons are responsible for far more deaths.”

Similar advice comes from the Brookings Institution. “[W]e should pledge to retaliate at a time and in a manner of our choosing, against regime assets used in indiscriminate attacks on civilians,” senior fellows Michael O’Hanlon and Steven Heydemann wrote in USA Today.

U.S. Sen. Lindsey Graham, R-S.C., appears to lean in the same direction. On Monday he wrote, “I hope that the Trump administration will not sit on the sidelines and watch the wholesale massacre of innocent men, women and children in Idlib. When it comes to Syria, we either act now or pay a heavy price later. To sit on the sidelines and not rally the world to the cause of Idlib would be a major foreign policy mistake. ... With American leadership we can stop this ongoing humanitarian disaster.”

While it is possible that the threat of U.S. attacks might deter Assad — and more importantly, persuade Russian President Vladimir Putin to tell Assad to call off the assault — upping the ante in Syria runs some clear risks that only President Trump can fully evaluate.

If Assad goes ahead with his bloody plans — as he has repeatedly done despite American warnings and even retaliation — then Mr. Trump might have to act to prevent a further massacre. That could bring him into a direct confrontation with Russia, which neither nation wants.

This is clearly a dangerous moment in the Middle East, where the United States and Russia came close to exchanging blows in 1973 during the Yom Kippur war. Mr. Trump wants to prevent a disaster in Idlib that could be ruinous for Turkey, Jordan and Western Europe, and deserves strong international support for demands that Russia and Syria call off their offensive. But Russia and Syria have repeatedly rejected such demands.

Unfortunately for the people of Idlib, the threat of U.S. retaliation may not be enough either.