He initially fought against the Romans during the First Jewish–Roman War as head of Jewish forces in Galilee, until surrendering in 67 CE to Roman forces led by Vespasian after the six-week siege of Jotapata. Josephus claimed the Jewish Messianic prophecies that initiated the First Jewish–Roman War made reference to Vespasian becoming Emperor of Rome. In response Vespasian decided to keep Josephus as a slave and presumably interpreter. After Vespasian became Emperor in 69 CE, he granted Josephus his freedom, at which time Josephus assumed the emperor's family name of Flavius.