The Syrian regime is determined to reconquer all of the territory it has lost. Aided by Russian bombers and Iranian troops, and emboldened by its success in terrorizing the populations of Ghouta and Daraa into submission, President Bashar al-Assad’s government is now preparing to attack Idlib, the last remaining province outside of his control. Idlib is home to some three million people, about half of them displaced, or forcibly evacuated, to the province from elsewhere. Many are crowded into unsanitary camps or sleeping in the open.

In recent days, regime troops have massed on Idlib’s border and leaflets have been dropped on residential areas calling on Syrians to accept “reconciliation” or face the consequences. Meanwhile, Russia has been sending reinforcements to its naval base in Tartus.

The Syrian troika — Russia, Iran and Turkey — designated Idlib a “de-escalation zone” last year. But what happens there next could potentially undermine the so-far mutually beneficial agreement among the three countries.

De-escalation in Idlib genuinely serves Turkey’s interests: It keeps both the Syrian Kurds and the Assad regime away from the border, it preserves Turkey’s relevance to a long-term settlement, and it houses Syrians who would otherwise try to join the 3.5 million refugees already in Turkey. Turkey has shown its commitment by setting up observation posts around the province and by establishing the National Liberation Front, an amalgam of Free Army and Islamist militias that follow Turkish orders. Russia and Iran, on the other hand, have always seen the de-escalation zones as tactical and temporary. Just as Daraa and Ghouta were abandoned, so (they hope) Idlib will be returned to Mr. Assad’s control.