The five calamities
According to the Mishnah (Taanit, 4:6), five specific events occurred on the ninth of Av:
-On this day, the Twelve spies sent by Moses to observe the land of Canaan. Two of the spies brought a positive report, but 10 of the spies brought an "evil report" about the land that caused the Children of Israel to cry, panic and despair of ever entering the "Promised land". For this, they were punished by G-d that they would not enter, and that for all generations the day would become one of crying and misfortune for the descendants of the Children of Israel, the Jewish people. (See Numbers ch 13-14)
Solomon's Temple (the First Temple) and the Kingdom of Judah were destroyed by the Babylonians led by Nebuchadnezzar in 586 BCE and the Judeans were sent into the Babylonian exile.
-The Second Temple was destroyed by the Roman Empire in 70 CE scattering the people of Judea and commencing a two thousand year Jewish exile.
-The Bar Kokhba's revolt against Rome failed, and Bar Kokhba was killed, as was Rabbi Akiva and many other important sages of the Mishnah, and Betar was destroyed.
-Following the Siege of Jerusalem, the subsequent razing of Jerusalem occurred one year later.
-According to the Talmud (Tractate Taanit), the destruction of the Second Temple began on that date and was finally consumed by the flames on the next day -- the Tenth of Av.
Later calamities on 9 Av
A large number of calamities are alleged to have occurred on the ninth of Av:
-The declaration of the Crusades by Pope Urban II in 1095
-The burning of the Talmud in 1242
In 1290, the signature of the edict by King Edward I expelling the Jews from England
-The Alhambra decree was put into effect, leading to the Jewish expulsion from Spain in 1492
-The First World War started in 1914
-The first killings at Treblinka took place in 1942
-The AMIA Bombing (Asociación Mutua Israelita Argentina) by Arab terrorists on July 18, 1994 in Buenos Aires, Argentina, which killed 86 and wounded more than 120.