Complex sentence: By this point, 40,000 to 50,000 civilian residents of Manbij had fled the city, although a number of locals remained to volunteer with the SDF and had begun training to fight ISIL.
Split into simpler sentences: By this point, 40,000 to 50,000 civilian residents of Manbij had fled the city. Several locals also had volunteered for the SDF and had begun training to fight ISIL.