~ Ceasing of the daily sacrificial offering at the Temple:
Every day in the Temple the Kohanim (priests) would offer two sheep as sacrifices, one in the morning and one in the evening. Even during the siege on Jerusalem, the practice continued. However when the walls of Jerusalem was breached the Kohanim were called to join the fight and none available to attend to the sacrifices. In addition, the daily sacrifice stopped due to the unavailability of cattle in Jerusalem during the siege.

 ~ The walls of Jerusalem were breached:
This event preceded the destruction of both holy temples, first by the Greek and later by the Romans.

 ~ Burning of a Torah Scroll and placing of an idol in the holy sanctuary of the Temple:
The Talmud in the Taanit Tractate relates that Apostmos, the Roman captain of the occupation forces, publicly burned a Torah scroll and placed an idol in the Temple on the seventeenth of Tammuz. It is also mentioned in Melachim (Kings) that Menashe, one of the kings of Israel, placed an idol in the Temple’s sanctuary on this date.

 ~ The conquering of Jerusalem by the first crusaders in the year 1099:
On the seventeenth of Tammuz of that year, the crusaders slaughtered Jews in Jerusalem and burned a synagogue and the Jews who were hiding inside it.

 ~ Confiscation of Talmudic Manuscripts:
Pope Gregory IX ordered that all manuscripts of the Talmud confiscated on the seventeenth of Tammuz.

Other Tragic Events On The Seventeenth Of Tammuz
 ~ Toledo, Jaen, Spain: In 1391 more than 4,000 Jewish people were murdered.
 ~ Prague: In 1559 the Jewish Quarter of Prague was looted and then burned.
 ~ Lithuania: In 1944 the Kovno Ghetto was liquidated
 

MazorGuide Recommended Reading

 - Discussion and Consolation: Experiencing the Churban ~ by: Rabbi Yisroel Reisman - A 6-cassette Set