The scene is this: some rabbis are discussing the events of around 70 CE (a generation or so after Jesus' death). Judea was under Roman occupation, and in 70, the Romans decided to take action to end the political trouble and fomenting rebellion in the province. This culminated in the destruction of the (second) Temple, the centre of Jewish worship, and the whole city of Jerusalem was also razed and the Jewish population relocated. This is one of the major tragedies that is commemorated by today's fast. R Jochanan tells the following story:
Once there was a man who had a good friend named Kamtza and a mortal enemy named bar Kamtza. He threw a party, and told his servant to invite his friend Kamtza, but the servant invited his enemy bar Kamtza. When the host saw bar Kamtza sitting there at his party, he was furious.
"What are you doing here?!", he cried. "Get out of my house!"
Bar Kamtza begged to be allowed to stay, offering to pay for whatever he ate and drank. No way, said the host. Bar Kamtza offered to pay half the cost of the party. Still no way. He offered to pay the whole cost of the party. No! The host had bar Kamtza seized and bodily thrown out of his house.
Bar Kamtza saw the rabbis, the community leaders, sitting at the party and watching this happen. None of them said anything, so bar Kamtza thought they must be ok with him being treated this way. He decided to become an informer and approached the Roman authorities. He had a report made to Caesar saying that the Jews were planning a rebellion. His Roman contact was cynical: "Sez who?". Bar Kamtza replied, "If you don't believe me, why don't you send them a sacrifice for the glory of the Empire, and see if they offer the sacrifice or not."
The Romans gave bar Kamtza a three-year-old calf for sacrifice. He took the calf and deliberately made a blemish in its mouth or eye, so that the Jews would consider the animal unfit for sacrifice but the Romans would still see it as acceptable for sacrifice. Now the Jewish community had a dilemma: should they sacrifice the blemished animal in the hope that the Romans would leave them alone, or should they follow Jewish law correctly and refuse to sacrifice it, risking terrible reprisals?
Most of the rabbis favoured sacrificing the blemished animal anyway, given the political situation. Or maybe they should refuse to sacrifice it and kill bar Kamtza so that he couldn't make his damaging report to the Roman authorities. R Zechariah the son of Avkoulos protested: they couldn't give the impression that they were the sort of people who went around sacrificing blemished animals, and they couldn't give the impression that they were the sort of people who would kill someone for a trivial offence like making a blemish on a sacrficial animal.
Because of this, the Temple was destroyed, and Jerusalem was burned, and the Jews were sent into exile.
So the question is, whose fault is it that Jerusalem was destroyed? Answers in comments please! Not doing a poll because I want to know your reasoning. I've heard it said you can deduce a lot about someone's character from whom they blame in this story.
(The story is my paraphrase of a chunk from Gittin 55b. And yes, some of you have played this one before. Oh, and R Jochanan says it's R Zechariah b Avkoulos' fault for being excessively pious, but that doesn't necessarily mean he's right.)