Laws of Public Fast Days
5. The Ninth of Av (Tish'oh B'Ov) [is also a public fast day]. On this day, it was decreed that our ancestors in the desert would not enter the land of Israel; the spies had returned that day [from their reconnaissance mission,] and, [in response to their report,] the Jews wept for no purpose(1); consequently, [G-d] established this as a day of weeping for [future] generations (2).
On this day, the ultimate destruction occurred, namely, the devastation of both the first and second Temples. Furthermore, the city of Beitar, a large metropolis with tens of thousands of Jewish inhabitants, was conquered on this day [by the Romans] (3). Also, it was on this day that Turnus Rufus ploughed the site of the Temple and thus fulfilled the prophecy [Michah 3:12]: "Zion will be ploughed like a field."
([The laws regarding] another communal fast, the Fast of Esther, are discussed in Chapter 141, Law 2).
(1) The report given by the spies about the Land of Canaan frightened the Jews, and convinced them that they would not have the power to conquer the land. In response to their lack of faith, G-d decreed that the generation would die out while wandering through the desert for 40 years, and only their children would be able to enter the land (See Numbers, Chapters 13 and 14).
(2) The Rambam (Yad, Laws of Fasts, 5:3) calls the ninth of Av a day "set aside for punishment" ("muchan le'poranus").
(3) Shimon Bar Kochba successfully led a rebellion against the Romans in about 128 CE, and re-established an independent Jewish state in the land of Israel (limited to the area of Judah) for a brief period. Many thought that he was the Moshiach (Messiah). In about 135 CE, the Roman army re-conquered Jerusalem, forcing Bar Kochba and the Jews to retreat to Beitar. When Beitar finally fell on the 9th of Av, hundreds of thousands of Jews were killed, and the Rambam (Yad, Laws of Fasts, 5:3) writes that the pain of the tragedy was equivalent to that of the destruction of the Beit Hamikdash.