We fast on Tishah B’Av because of five tragedies that occurred to the Jewish People on that date:

1. As a result of the sin of the meraglim (spies) Hashem decreed on Tishah B’Av that our forefathers would have to wander for forty years in the desert before entering the land of Israel. The men of that generation who were twenty years old and above would die in the wilderness and not enter Eretz Yisrael. 

2. The first Bet HaMikdash was destroyed.

3. The second Bet HaMikdash was destroyed.

4. Beitar, a large city in Israel was captured and fell to the Romans on Tishah B’Av (52 years after the second Hurban in the time of Rabbi Akiva), and hundreds of thousands of Jews died — including Bar Kochba. At first Bar Kochba — the name given to Shimon ben Kosibah — was a legendary and strong leader of the Jewish revolution against Rome and was supported by everyone, including Rabbi Akiva who said he was worthy of being Mashiah. He was accepted as the king of Judah. Hadrian, the king of Rome, sent a large army to lay siege and put an end to the independent Jewish state. For almost three years the Jewish commonwealth survived — but after one year Jerusalem, their capital was captured and placed under Rome’s rule. Bar Kochba and his followers, 400,000 strong, went to the fortress city of Beitar planning to continue fighting. During this period Bar Kochba’s behavior changed and he became erratic and a despot. As a result Rabbi Akiva and the other Rabbis with whom Bar Kochba argued (because he suspected that they were disloyal) could no longer support him. Rabbi Elazar HaModa’ie, Bar Kochba’s uncle and a great scholar, lived in Beitar and he prayed constantly, fasted and wrapped himself in sackcloth so that the city would not fall to the Romans. Hadrian was informed about this Rabbi and was told that as long as he prayed the Romans would be unsuccessful. To cast suspicion upon Rabbi Elazar this same informer went to the sage and whispered in his ear, making sure that others would see and suspect him of treason. Sure enough false accusations were brought against Rabbi Elazar to Bar Kochba. Despite Rabbi Elazar’s denial that nothing was said to him and he was not involved in any plot Bar Kochba kicked him and he died. Shortly after this horrible act Beitar fell and hundreds of thousands were killed. The Gemara describes the horrible death and devastation of Beitar — there was so much blood that it reached the nostrils of the horses; the flow of blood was so great that it moved heavy rocks more than 4 mil and 500,000 children were wrapped in sefarim and burned. It was a most tragic calamity, one of great magnitude. 

5. Turnus Rufus plowed the area of the Bet HaMikdash and its surroundings.