The destruction of Jerusalem began with a dinner party. According to Gittin 55b, a man threw a banquet and sent his servant to invite his friend Kamtza. The servant brought Bar Kamtza instead—the host's enemy.
When the host saw Bar Kamtza at his table, he was furious. "Get out," he said. Bar Kamtza, humiliated in front of the city's elite, tried to save face. "I will pay for my portion of the meal. Just do not embarrass me." No. "I will pay for half the feast." No. "I will pay for the entire feast." No. The host grabbed Bar Kamtza by the hand, stood him up, and threw him out.
The Sages were at the banquet. They watched the entire scene. They said nothing.
Bar Kamtza drew the obvious conclusion: since the rabbis did not protest, they approved of his humiliation. He went to the Roman emperor and told him the Jews were rebelling. The emperor asked for proof. Bar Kamtza said: "Send an offering for the Temple and see if they sacrifice it."
The emperor sent a choice calf. On the way to the Temple, Bar Kamtza made a small blemish on the animal—on its upper lip, or some say its eyelid. The blemish was significant under Jewish law but invisible under Roman standards. The priests could not sacrifice it. They also could not explain why to the Romans.
The Sages considered sacrificing the blemished animal anyway, to keep the peace. Rabbi Zekharya ben Avkolas objected: "People will say blemished animals may be offered." They considered killing Bar Kamtza. Rabbi Zekharya objected again: "People will say blemish-makers deserve death."
So they did nothing. Bar Kamtza reported to Rome. The war began. As the verse warns: "Because of the wrath of the Lord it came to pass in Jerusalem and Judah, until He had cast them out from His presence" (II Kings 24:20). Rabbi Yohanan later said: "The excessive humility of Rabbi Zekharya ben Avkolas destroyed our Temple, burned our Sanctuary, and exiled us from our land."