In the famous story about the meeting between Rabban Yochanan Ben Zakai and Vespasian, RYb"Z brings an asmachta from a verse to show that Vespasian will become emperor. The verse is:
"And the Lebanon shall fall by a mighty one" (Yeshayahu 10:34)
RYb"Z explains that "a mighty one" is a king. My question is: How does this show that Vespasian will be emperor? After all, when news of his imperial status came, Vespasian left Judea in the hands of his son Titus (and this is logical because the emperor couldn't stick around in a rebellious backwater province). This means that the situation was exactly as it was before Vespasian left: A general who is not an emperor leading the assault on Yerushalayim. So either way, the Temple would not have been destroyed by an emperor. So what does the verse prove, or in what manner does it strengthen RYb"Z's claim?