Tisha B'Av is all about the remembrance of the major tragic events that happened to the Jewish people in the past. If there were no such events, there wouldn't have been any need to observe Tisha B'Av. The first event was when Moses sent twelve of his people to go and spy on the land of Canaan which according to them was the land God had promised them. But ten people returned with bad news which further threw the Israelites into more confusion and agony, although Joshua and Caleb came back with positive reports.
The second event was about the demolition and the destruction of the first temple of Jerusalem which was built by King Solomon. The third event was the demolition and destruction of the second temple of Jerusalem which was built by Ezra and Nehemiah. The first event was when the Romans captured the fortress city of Betar and the chrusing of Bar Kokhba's revolt which led to the death of over five hundred thousand Jewish people.
Two main events took place that led to the Jewish people considering this to be a day of remembrance. The first one is the destruction of Solomon’s Temple. This took place during the Neo-Babylonian Empire. In ancient Jerusalem, the Jewish people would worship in Solomon’s Temple, which was also known as the First Temple or the Holy Temple. Nebuchadnezzar II ruled this area, and there was a siege that took place in 587 BCE.
As a result of the blockade, this ruler then destroyed the Jewish people’s temple. After this took place, the temple was rebuilt. It was called the Second Temple. The Romans destroyed this temple in 70 CE. This occurred due to a revolt by the Jewish people. Therefore, the Jewish people now remember this day for these two disasters.