“…and the Lebanon shall fall through a mighty one.” (Yeshayahu 10:34) R’ Yochanan ben Zakkai said to Vespasian ‘peace unto, oh king! Peace unto you, oh king!’ He replied ‘you have made yourself twice-liable for death. First, because you called me king and I am not a king.
Second, because if I am a king, then why have you not come to me before now?’ R’ Yochanan said back to him ‘that which you said, that you are not a king – if you are not a king then Jerusalem would not have been handed over to you, as it says “…and the Lebanon shall fall through a mighty one.” A mighty one only refers to a king, as it says “And their prince shall be from them…” (Yirmiyahu 30:21) And Lebanon only refers to the Holy Temple, as it says “…You are as Gilead to me, O head of the Lebanon…” (Yirmiyahu 22:6) and it is written “this good mountain and the Lebanon.” (Devarim 3:25) And why is it called Lebanon? Because it bleaches away (malbin) the sins of Israel.