A surge of rebel gains in Syria is overturning long-held assumptions about the durability of President Bashar al-Assad’s regime, which now appears in greater peril than at any time in the past three years.

The capture Saturday of the town of Jisr al-Shughour in northern Idlib province was just the latest in a string of battlefield victories by rebel forces, which have made significant advances in both the north and the south of the country.

As was the case in the capital of Idlib province last month, government defenses in Jisr al-Shughour crumbled after just a few days of fighting, pointing as much to the growing weakness of regime forces as the revival of the opposition. […]