The Sages of the Mishnah state, “Five (tragic) events befell our forefathers on the seventeenth of Tammuz: the Tablets [containing the Ten Commandments] were broken, the continuous daily offering (Tamid) was terminated, the city [of Jerusalem] was breached, Apostomus burnt a Torah scroll, and [someone] erected an idol in the Holy Temple” (Ta’anit 26a-b).
[The breaking of the Tablets]: After the revelation at Mount Sinai and the [Jewish people’s] acceptance of the Torah, Moshe Rabbeinu remained on the mountain for forty days and forty nights, where he learned Torah directly from God. When he descended the mountain carrying the covenantal Tablets, which were engraved with the Ten Commandments, he saw that some of the people had strayed after a golden calf that they had fashioned. His strength immediately failed and he broke the Tablets. Thus, not only were the Tablets broken on the seventeenth of Tammuz, but the sin of the Golden Calf occurred on that day, as well.
The termination of the Tamid offering: The Tamid is the most important sacrifice offered in the Holy Temple. Its importance stems from its consistency: they would offer the Tamid [twice a day], every day, once in the morning and once in the evening. During the first siege on Jerusalem, the Romans supplied [the Jews] with lambs to be used for the Tamid offering. This went on until the sixteenth of Tammuz. The seventeenth of Tammuz was the first day they failed to offer the Tamid(see Bava Kama 82a).
The burning of the Torah by Apostomus: He was a [wicked] Roman officer.
Erecting an idol in the Holy Temple: Some say that this happened in the First Temple, and [King] Menasheh was the one who did it. Others say that it took place in the Second Temple era, and the perpetrator was the wicked Apostomus (Yerushalmi, Ta’anit 4:5).
However, the event that ultimately prompted [the prophets/rabbis] to establish a fast day was the fifth tragedy, the breaching of Jerusalem’s walls. The Romans besieged the city for three years, yet they were unable to conquer it. Finally, because of baseless hatred and infighting [among the Jews], the city’s defenders grew weak and the Romans prevailed. On the seventeenth of Tammuz, they succeeded in breaking through the walls of Jerusalem and penetrating inward. Practically speaking, once the city was breached, the campaign was lost. Battles continued to rage in Jerusalem for another three weeks, but in the end, [the Romans] conquered the Temple mount and burned down the Second Temple, on the ninth of Av, thus beginning the long exile.
If we ponder the matter, we will see that there is a deep connection between the five tragedies that occurred on the seventeenth of Tammuz. Each one reveals a crisis that affects the spiritual roots first, then introduces cracks in the wall of faith, and eventually causes serious damage, which if not repaired promptly will lead to the total destruction of Tish’a B’Av. When the Jews committed the Sin of the Calf, they did not completely reject God. They still believed in HaShem, creator of the world; they just believed that the Calf had certain powers, as well. Once they began committing idolatry, however, they had no strength to resist the council of the Spies, and they rebelled against HaShem and His servant Moshe, thus violating the purpose for which the Jewish nation was created – the revelation of the Shechinah in this world, in the Land that was designed for this, Eretz Yisrael. The same is true of the termination of the Tamid offering, the placement of an idol in the Temple, and the burning of the Torah. None of them, as of yet, entail [complete] destruction, but they represent a fundamental spiritual rift, which if left unrepaired will grow worse and worse, leading eventually to total destruction.
. See previous note regarding the breaching of Jerusalem’s walls during the first churban, which Scriptures say occurred on the ninth of Tammuz, and why we fast on the day it happened during the second churban. See also Torat HaMo’adim (Yosef), p. 7, where the author cites varying opinions as to when the idol was erected in the Temple and when the Tamid was terminated.