In addition to his battle-weary forces, Obregón also called in his reserve, which pushed the Villistas back to their own lines. In a stroke of good fortune for Obregón, the Villistas’ supply of ammunition for their small arms had run low after the day's fighting. In the middle of the retreat, one of Villa's commanders defected to the Constitutionalists and opened fire on Villa's troops. By a series of good tactical decisions and considerable luck, Obregón had won the first battle of Celaya.