SARJEH, Syria – Rebel commander Ahmed Eissa al-Sheikh keeps a paper on his desk bearing the names of the dead from his brigade. The first 16 are neatly typed below a Quranic verse extolling martyrdom. The next 14 are handwritten and crammed into the margin, because the paper is full.

Al-Sheikh, an Islamist with a long black beard and gray fatigues, runs the Falcons of Damascus group from the mayor's office in his village, which his fighters have taken over. The list is a constant reminder of al-Sheikh's personal score with the Syrian regime: 20 of the dead are his relatives, including three brothers and his 16-year-old son, all killed fighting Syrian forces in the last year.

One of northern Syria's most powerful and best-armed commanders, Al-Sheikh boasts more than 1,000 fighters, and they don't shy away from rougher tactics themselves. They have released prisoners in bomb-laden cars and then detonated them at army checkpoints – turning the drivers into unwitting suicide bombers.