The Talmud's Taanit 29a says that Tisha B'Av is a day of mourning because God metaphorically said, 'Since you cried for no reason, I'll give you something to cry about!' . Tisha B'Av commemorates five calamities that the Jewish nation has experienced: 

• The sin of the spies 
Before the Israelites entered the land of Israel, 12 spies explored the land and brought a bad report to Moses, causing the Jews in the desert to refuse to enter. 



• Destruction of the First Temple 
In 586 BCE, the Babylonians, led by Nebuchadnezzar, destroyed the First Temple built by King Solomon. The Talmud says the destruction began on the Ninth of Av and continued throughout the Tenth of Av. 



• Destruction of the Second Temple 
In 70 CE, the Romans destroyed the Second Temple built by Ezra and Nehemiah. 



• Destruction of Beitar 
In 135 CE, the Romans suppressed the Bar Kokhba revolt and destroyed the city of Beitar, killing over 500,000 Jews. 



• Plowing of the Temple Mount 
Roman general Turnus Rufus plowed the Temple area in Jerusalem, rebuilding the city as a pagan city. 



לָכֵן֙ בִּגְלַלְכֶ֔ם צִיּ֖וֹן שָׂדֶ֣ה תֵֽחָרֵ֑שׁ וִירוּשָׁלַ֙͏ִם֙ עִיִּ֣ין תִּֽהְיֶ֔ה וְהַ֥ר הַבַּ֖יִת לְבָמ֥וֹת יָֽעַר׃ {פ}

Assuredly, because of youZion shall be plowed as a field,Jerusalem shall become heaps of ruinsAnd the Temple Mount a shrine in the woods.

Rabbi Yosef Bitton writes: . The least known of these tragedies is also one of the most relevant, as in some way, the relevance of this act is still present. 

I refer to the 'plowing of the city' of Jerusalem (חרישת העיר) in the year 130 CE. To understand why the city was plowed—and what this meant at the time—we must review what happened after the Romans destroyed the Second Temple (year 68 CE).