Which event caused Yohanan ben Zakkai to start a school for rabbis?

Yohanan Ben Zakkai was able to put up a school for rabbis due to the permission of Vespasian, a Roman general. When the city where Zakkai lived was under siege by Vespasian, he faked his death so that he can hide inside a coffin and his pupils can bring him out of the city. He told Vespasian that he had a vision of the general would soon be an emperor, and he requested that his city be spared from war and for him to be able to build a school. Vespasian agreed that if the prophesy came true, he will grant his request. After becoming the emperor, Ben Zakkai was able to put up his school.