The Sages of the Mishnah state, “Five [tragic] events befell our forefathers on the ninth of Av: it was decreed upon our ancestors that they would not enter the Land, the First and Second Temples were destroyed, [the city of] Beitar was captured, and the city [of Jerusalem] was ploughed over” (Ta’anit 26b).
The first event happened in the Generation of the Wilderness. Moshe granted the nation’s request and sent twelve spies to survey the Land of Canaan. When they returned, ten of them slandered the Land and “melted the hearts” [cf. Devarim 1:28] of the people, saying that they could not conquer the Land of Canaan because its inhabitants were strong and gigantic. The entire congregation lifted up their voice, and the people wept that night. All the children of Israel complained against Moshe and Aharon, and the entire congregation said to them, “If only we had died in the land of Egypt, or if only we had died in this wilderness! Why is the Lord bringing us to this Land to die by the sword? Our wives and our children will be for prey. Is it not better for us to return to Egypt?” And they said to one another, “Let us appoint a leader and let us return to Egypt”(BeMidbar 14:1-4).
Granted, Yehoshua and Calev rebuked them, saying, The Land is very, very good. If the Lord desires us, He will bring us to this Land and give it to us, a Land that flows with milk and honey. Only, do not rebel against the Lord, and do not fear the people of the Land, for they are [like] our bread; their protection has left them, and the Lord is with us; do not fear them(ibid. 14:7-9). Nonetheless, the people did not accept their words. On the contrary, The entire congregation said to stone them with stones(v. 10).
The Sin of the Spies was worse than the Sin of the Calf on many levels. When the Jews made the Golden Calf, they did not completely reject God and Moshe; they merely erred, thinking that Moshe disappeared and that HaShem would no longer reveal Himself to them manifestly. Therefore, they felt it necessary to search for a “god” that could serve as an intermediary between themselves and the Creator. Consequently, the Holy One, blessed be He, forgave the Jews for this sin. When it came to the Sin of the Spies, however, the Jews denied God’s ability to operate in this world and help them conquer the Land. They also betrayed the main mission for which the world was created and the Jews were chosen – to reveal God’s Shechinah in this world, by way of Eretz Yisrael. Therefore, HaShem did not pardon the Sin of the Spies; rather, He decreed that all those who participated in the sin must die in the desert. Only Yehoshua bin Nun and Calev ben Yefuneh, who refused to join in the sin, were privileged to enter the Land.
That night, when the congregation wept and showed disdain for the Desirable Land, was the night of Tish’a B’Av. HaKadosh Baruch Hu said, “You wept in vain; I will establish for you weeping for all generations” (Sanhedrin 104b). At that moment, it was decreed that the Beit HaMikdash would be destroyed (Midrash Tanchuma, Shelach).
The Sin of the Calf caused a breach in the wall of faith; consequently, the walls of Jerusalem were breached, and the crack [that had formed] in the Torah’s glory and the Temple service split asunder. The Sin of the Spies destroyed the fundamental belief in Israel’s mission to sanctify God’s name in this world, and all of its resultant troubles signify the nullification and destruction of our ability to consistently reveal holiness in the Land. First, it was decreed, on Tish’a B’Av, that the Generation of the Wilderness would not enter the Land, and since we did not subsequently rectify the Sin of the Spies, both Temples were destroyed. We failed to rectify the sin after that, as well, so the city of Beitar was destroyed when Bar Kochva’s rebellion faltered, and Jerusalem was ploughed over. All of these tragic events prevented the Shechinah from being revealed in the Land, and that is why we mourn and fast on Tish’a B’Av.