On the 9th of Av throughout history many tragedies befell the Israelites/Jewish people, including:
- The incident of the spies slandering the land of Israel with the subsequent decree to wander the desert for 40 years.
- The destruction of the first Temple in Jerusalem by Nevuchadnetzar, King of Babylon in 423 BCE.
- The destruction of the Second Temple in Jerusalem by the Romans in 70 CE.
- The fall of Betar and the end of the Bar Kochba revolt against the Romans 65 years later, 135 CE.
- Pope Urban II declared the First Crusade. Tens of thousands of Jews were killed, and many Jewish communities obliterated.
- The Jews of England were expelled in 1290.
- The Jews of Spain were expelled in 1492.
- World War One broke out on Tisha B'Av in 1914 when Russia declared war on Germany. German resentment of the Treaty of Versailles set the stage for World War II and the Holocaust.
- On Tisha B'Av, deportation began of Jews from the Warsaw Ghetto.
Info from Aish.com