The Sages state in the Mishna:

Five tragic events befell our forefathers on the ninth of Av: It was decreed that our ancestors would not enter Eretz Yisrael; the First and Second Temples were destroyed; Beitar was captured; and the city [of Jerusalem] was plowed up. (Ta’anit 26b)

The first event happened in the generation of the wilderness. Moshe granted the people’s request and sent twelve spies to scout the Land of Canaan. Upon their return, ten of them slandered the land and “took the heart out of” the people (Devarim 1:28) by saying that they would not be able to conquer the Land of Canaan because its inhabitants were powerful giants.

The whole community broke into loud cries, and the people wept that night. All the people of Israel complained against Moshe and Aharon. The whole community said to them: “If only we had died in the land of Egypt, or if only we might die in this wilderness! Why is the Lord taking us to that land to fall by the sword? Our wives and our children will be spoils! It would be better for us to go back to Egypt!” And they said to one another, “Let us head back for Egypt.” (Bamidbar 14:1-4)

Yehoshua and Kalev rebuked them, saying:

The land…is exceedingly good. If the Lord is pleased with us, He will bring us into that land, a land that flows with milk and honey, and give it to us; only you must not rebel against the Lord. Have no fear then of the people of the country, for they are our prey: Their protection has departed from them, but the Lord is with us. Have no fear of them! (ibid. 7-9)

Nonetheless, the people did not accept their words. On the contrary, “the whole community threatened to pelt them with stones” (ibid. 10).

The Sin of the Spies was worse than the sin of the Golden Calf on many levels. In the sin of the Golden Calf, the people did not completely reject God and Moshe; they merely erred, thinking that Moshe disappeared and that God 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 them and the Creator. Consequently, God later forgave the Jews for this sin. When it came to the Sin of the Spies, however, the people denied God’s ability to operate in this world and help them conquer Eretz Yisrael. They even betrayed their main mission, for which the world was created and for which Israel was chosen – to reveal the divine Presence in this world, through Eretz Yisrael. Therefore, God 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 b. Nun and Kalev b. Yefuneh, who refused to join in the sin, were privileged to enter Eretz Yisrael.

That night, when the people wept and showed disdain for the “delightful land” (Zechariah 7:14), was the night of the ninth of Av. God said, “You wept in vain; I will make you weep for all generations” (San. 104b). At that moment, it was decreed that the Temple would be destroyed (Midrash Tanĥuma, Shelaĥ).

The sin of the Golden Calf caused a breach in our wall of faith; consequently, the walls of Jerusalem were likewise breached, and a crack formed in the Torah’s glory and the Temple service. The Sin of the Spies destroyed basic faith in Israel’s mission to sanctify God’s name in this world, and all of the hardships that stemmed from it indicate the nullification and destruction of our ability to reveal holiness in Eretz Yisrael on a permanent basis. First, it was decreed, on the ninth of Av, that the generation of the wilderness would not enter Eretz Yisrael, 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 Kokhba’s rebellion faltered, and Jerusalem was plowed up. All of these tragic events prevented the revelation of the divine Presence in Eretz Yisrael, and that is why we mourn and fast on Tisha Be-Av.