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Turkey's nightmare is coming true.

Not only is the battle for Aleppo sending tens of thousands of desperate people fleeing toward Turkey, but the fall of the rebel-held city would deliver a major blow to Ankara's Syria policy.

Syrian government forces, backed by Russian air power, have cut off the last major supply route to rebels in Aleppo. The route, known as the Azaz corridor, links rebel-held eastern Aleppo with Turkey.

If the corridor falls, the rebels could lose Aleppo -- and the entire Turkey-Syria border could fall under the control of forces that Ankara hates: the forces of President Bashar al-Assad's Russian-backed regime, and the Kurds.

Russian involvement has altered the course of the Syrian civil war for Turkey. Just a few months before Russia entered the war, Turkey and the United States had agreed on the outlines of a de facto "safe zone" along the Turkey-Syria border.

The deal was expected to significantly increase the scope of the U.S.-led air war against ISIS in northern Syria.

The agreement aimed to drive the so-called Islamic State out of a 68-mile-long area west of the Euphrates River and reaching into the province of Aleppo.

Ankara hoped that with the deal, the Kurdish expansion in the north would stop, the Syrian opposition would gain ground and fully capture Aleppo, and the Assad regime would be weakened. Turkey even reportedly trained a militia that would be tasked with policing the safe zone.