BEIRUT — Syrian rebels seized a sprawling military base in a lightning assault Tuesday, marking the latest in a series of defeats for President Bashar Assad’s increasingly embattled forces, rebels and activists said. Assad’s forces have suffered a stunning string of setbacks over the past three months at the hands of insurgents, members of al Qaeda’s local affiliate and the Islamic State extremist group. The Western-backed rebel alliance known as the Southern Front led the dawn assault on the army base in the southern Daraa province, known as Brigade 52, said Ahmad al-Masalmeh, an opposition activist in Daraa. [...] state-run news agency SANA said army units struck “terrorist” groups in Daraa province. The Free Syrian Army’s Southern Front is an alliance of rebel groups backed by the U.S.