BEIRUT, Lebanon — The defection of a young general close to President Bashar al-Assad of Syria has provided the most telling sign yet of eroding support for his government among even the most elite and trusted Sunni Muslims, who serve as a critical pillar of the security forces and civilian administration.

But while the defector, Brig. Gen. Manaf Tlass, gained world attention when he fled Damascus on Thursday, President Assad’s bigger military challenge is the swelling number of silent objectors — soldiers of all ranks lacking the means to flee, or the interest, but no longer cooperating with the government. Instead of responding to the call to duty, they are staying home, abandoning their posts as the opposition grows bolder, stronger and more effective, said Syrian military experts and defectors.

Mr. Assad’s loyal inner circle and core support remains the Alawite community, a minority Muslim sect. But Alawites constitute no more than 12 percent of the 23 million population, so the Assad family has for decades relied on the majority Sunnis for their legitimacy and practical support. Sunnis make up the bulk of the nation’s foot soldiers, hold posts throughout the bureaucracy and dominate the elite in the business community.

A few Sunnis have always held high-profile positions in the government and military. General Tlass’s father, Mustafa, like his son a Sunni, was a confidant of the president’s father, Hafez, and served as defense minister for 32 years under both men.