The world has seen this scenario before. Syrian President Bashar Assad launches an offensive against a rebel stronghold, and in the process legions of civilians die — either by poison gas, barrel bombs, or burial beneath the rubble of razed buildings. Now Assad, with the help of his Russian and Iranian allies, is poised for an assault on the rebels’ last swath of territory, the northwest Syrian province of Idlib. This siege, like others before it, portends a massacre of innocents.

The Trump administration has warned Assad that the U.S. would take military action — it didn’t say what kind — if he again kills his own people with chemical weapons. The Syrian dictator’s use of chemical weapons in 2017 and again last April killed scores of civilians. After each attack, President Donald Trump ordered strikes on Syrian military bases. It’s doubtful that those punishments have deterred Assad; according to The Wall Street Journal, U.S. officials say Assad has endorsed the use of chlorine gas in his Idlib offensive.

Can the U.S. change the trajectory of a potential humanitarian catastrophe in Idlib? Almost certainly not. Unfortunately, Washington scurried to the sidelines of the Syrian conflict long ago — a move that limits U.S. leverage in Syria now.

The U.S. floundering in Syria began when President Barack Obama warned that Assad would be crossing a red line if he used chemical weapons against his own people. Assad ignored the warning, and Obama didn’t react militarily. Washington’s reticence let Russian President Vladimir Putin assert the Kremlin’s influence over the conflict. Today, Assad remains in power with the backing of Russia and Iran, which now have footholds on this crucial Mideast turf.

Trump inherited the chaos in Syria. He streamlined the U.S. mission there, focusing primarily on the defeat of the Islamic State. That goal has been largely accomplished — Islamic State was routed out of its de facto capital in Raqqa, and its presence in Syria now has shrunk to remote areas near the Iraq border.

The looming crisis in Idlib poses a new quandary for the Trump White House. Rebels numbering in the thousands are mixed in with a civilian population estimated at 3 million, one-third of them children. Many of the civilians are there because they fled other parts of war-torn Syria. At a recent United Nations gathering, U.S. ambassador Nikki Haley called the planned offensive “a playbook of death.”

Can the U.S. do more than denounce Assad? The U.S. still has 2,200 troops in northeast Syria. That preserves some leverage for the U.S. in the face of Russia and Iran’s presence in Syria. Turkey, which also has troops in Syria, strongly opposes the upcoming offensive in Idlib, primarily because the assault would trigger another wave of refugees into its country. Relations between Washington and Ankara have bottomed out, but the two capitals have a common goal in discouraging an Idlib assault — perhaps by pushing Putin to restrain his client Assad.

That’s not a satisfying or probably successful gambit. But having surrendered influential roles in Syria to Moscow and Tehran, that’s about all Washington can do for now.