BEIRUT, Lebanon — Near Damascus, Hezbollah fighters are defending the shrine of Sayida Zeinab, revered by Shiite Muslims, from encroaching Sunni rebels. To the north, they are not just part of a Syrian government drive to take back the strategic town of Qusair, rebels say, they are leading it.

On Tuesday, Syrian opposition activists reported that rebels had killed 15 Hezbollah fighters in Qusair. But they also said the rebels were besieged in the town and in danger of losing it, after black-clad Hezbollah fighters swept them from surrounding villages.

Recruited and trained to battle Israel and defend Lebanon, Hezbollah’s Lebanese Shiite guerrillas are pushing more and more deeply into a very different fight: in neighboring Syria, against fellow Arab Muslims trying to topple President Bashar al-Assad. Their leaders have made it increasingly clear that Mr. Assad’s war is their war, too.

Though Hezbollah has never been militarily stronger — it has more than replenished the weapons and fighters it lost in its brief war with Israel in 2006 — it finds itself in an unaccustomed situation. It is struggling to preserve credibility at home and fend off an array of new challenges abroad as it fights to preserve Mr. Assad’s rule, and the crucial arms pipeline he provides.