BEIRUT, Lebanon — When Syrian government forces on Sunday swept into Yabrud, a town long held by rebels near the Lebanese border, it was a symbolic turning point for insurgents and government supporters alike in a conflict now heading into its fourth year.

Yabrud was a rallying point for the government and its allies in the Lebanese militia Hezbollah, who were instrumental in the fight, just as they were when they helped take another crucial border town, Qusayr, last spring. Thirteen Syrian nuns had been held hostage by insurgents in Yabrud until last week, and the government had long said that the town harbored a factory that was making the car bombs that have killed scores in southern Beirut in recent months.

For Syrian opposition activists and some rebels, Yabrud had been a model of what they had hoped for from the uprising that began with peaceful protests in March 2011, a dream that seems to be receding. Before foreign fighters and Syrians with a more radical bent arrived in greater numbers late last year, it was a place where civilians, not fighters, held sway. Residents had governed themselves: collecting trash, keeping order and sheltering thousands of displaced Syrians. Christians and Muslims still lived together, and a negotiated understanding with government forces kept the town relatively unscathed.

But now the news from Yabrud is likely to deepen the despair of Syria’s opposition. A month of bombardments by the military exacted a heavy toll on the rebels there, who were also weakened by the failure of the exile opposition to unite and support the fighters, while an influx of jihadists undermined their claims to moderation and made them more of a target.