Syrian President Bashar al-Assad, who four months ago seemed on the verge of defeating rebel forces, is now mired in defensive battles on several fronts, complicating efforts to fight the Islamic State militant group.

In a span of weeks, the Islamic State has overrun military bases in Syria's east. In the west, the regime faces a coalition of rebels that threatens the heartland of Mr. Assad's Alawite minority and could alter the course of Syria's multi-sided civil war. Alawites, a Shiite-linked group that forms the backbone of the regime and pro-government militias, are angry over the loss of hundreds of troops last month after the Islamic State captured an air base in the northeastern province of Raqqa.

These developments come as President Barack Obama prepares to describe on Wednesday his own plan to defeat the Islamic State, a Sunni-extremist group also known as ISIS or ISIL. Mr. Assad's troubles could complicate the fight against ISIS by worsening a power vacuum that has allowed the Islamist group to thrive in Syria and neighboring Iraq.

These same conditions, however, have also raised hope that pressure will drive the Assad regime to the negotiating table, tamping down the civil war long enough to concentrate efforts on defeating the Islamic State, a threat to both the regime and Syrian rebels.

United Nations and Western officials hope to push tentative truce negotiations now under way between the regime and rebels. The idea is to persuade both sides—as well as their regional backers, Iran for the regime; Qatar, Saudi Arabia and Turkey for the rebels—that no one can win the war, these officials said during interviews in Syria and Lebanon.

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