In 1970, war broke out between the previously allied Jordanian government under King Hussein and the PLO led by Yasser Arafat. This conflict became known as the Black September, and resulted in the desertion of many Jordanian soldiers of Palestinian descent. Mahmoud was one of those who sided with the PLO, and defected in Jerash. He was appointed deputy commander of the PLO's so-called Yarmouk Forces. Serving under Saad Sayel, he consequently battled against his former comrades in the Royal Jordanian Army. The PLO was defeated, and ousted from Jordan by mid-1971. Mahmoud relocated to Syria, where he took part in Fatah's third conference at Hamouriyah in September 1971. He was appointed member of the Revolutionary Council of Fatah during this conference, and promoted to commander of the Yarmouk Forces in 1972. He consequently assumed command of PLO troops stationed in Mount Lebanon and the Beqaa Valley. As experienced military engineer, he was responsible for the construction of fortifications for Palestinian militants.