THE HORRORS of Syria’s civil war diminished somewhat in recent months after eight years of carnage and atrocities. But they could resume on a horrifying scale in the province of Idlib, which is home to some 3 million people and remains under the control of rebel groups. Late last month, a truce governing the region was ruptured when the regime of Bashar al-Assad launched a new offensive, in concert with Russian air power. Once again, bombs rained down on Syrian homes, hospitals and food stores; according to the United Nations, more than 150,000 people fled northward, toward Syria’s border with Turkey.

The attack threatens to trigger a humanitarian catastrophe larger than any Syria has yet seen. Tens of thousands of the civilians in Idlib are refugees from other parts of the country. In many cases, they were bused there by the Assad regime under deals to obtain the surrender of other rebel towns. An attempt to retake the province by force would trigger a massive new wave of refugees that could swamp Turkey — and perhaps extend to Europe, which is still suffering the political aftershocks from the mass arrival of Syrians in 2015.

Considering the stakes, the response of the Trump administration has been remarkably muted. President Trump has said nothing about the new crisis. When Secretary of State Mike Pompeo raised the subject during his meetings with Russian President Vladi­mir Putin and Foreign Minister Sergei Lavrov earlier this week, he was told that Moscow’s objectives were limited and included expanding a buffer zone to protect a Russian air base that has come under attack.