WESTERN GOVERNMENTS have been exhausted by the seemingly endless civil war in Syria. Many are distracted by polarizing domestic political conflicts. The result is a tragically muted reaction as the Syrian war approaches a horrific climax in the northern province of Idlib.

According to the United Nations, some 900,000 people have fled a new offensive by forces of the regime of Bashar al-Assad and its Russian and Iranian allies since December. That includes an estimated 500,000 children. Most of these civilians are crammed into a narrow strip of territory near the Turkish border, which is sealed. Many have no shelter from bitterly cold weather. “Mothers burn plastic to keep children warm,” U.N. official Mark Lowcock said last week. “Babies and small children are dying because of the cold.”

Russian and Syrian planes have deliberately triggered this exodus by relentlessly bombing civilian targets in Idlib, which has been the largest remaining stronghold of anti-Assad forces. The dominant rebel groups in the area are extremists with links to al-Qaeda and are regarded as terrorists by Western governments. But the offensive has made no distinction between them and the more than 3 million civilians in the province, hundreds of thousands of whom are refugees from other parts of Syria.

The only defender of these desperate people is Turkey, which has dispatched thousands of troops, tanks and artillery into the region in recent weeks. Turkish President Recep Tayyip Erdogan has threatened to launch an offensive against the Assad forces unless they pull back by the end of this week. Already there have been heavy clashes between Turkish and Syrian troops in which dozens have been reported killed. According to the Syrian Observatory for Human Rights, Russian and Syrian aircraft this week were bombing Turkish positions in the southern Idlib countryside, while Turkish artillery was supporting an offensive by rebel fighters.

Talks between Turkey and Russia resumed on Wednesday, and Mr. Erdogan appears to be hoping he can establish a safe zone stretching some 20 miles from the border and including the provincial capital, Idlib city. That would allow civilians to receive humanitarian aid and prevent the refugees from spilling into Turkey, which already harbors 3.6 million Syrians. The Russian government seems inclined to allow some kind of safe zone; at stake for President Vladimir Putin are the warm relations he has fostered with Mr. Erdogan, at the expense of Turkey’s alliance with the United States and NATO.

The Assad regime, however, is bent on recapturing all of Idlib, whatever the human cost. And the relentless pattern of the Syrian war has been unfulfilled Russian promises to control Damascus. That’s why it is urgent that the United States and European governments apply concerted pressure on Mr. Putin now to curtail the offensive, enforce a cease-fire and allow full humanitarian access to Idlib. They should make clear to Moscow that it will pay a substantial price — in the form of new sanctions and a reversal of European moves toward better relations — if the carnage does not cease.

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