Mourning has Broken
Ultra-Orthodox Jews mourn the destruction of the Temples in Jerusalem
(The First Temple was destroyed by the Babylonians in 586 BCE when they sacked the city. The second Temple [Herod's Temple] was destroyed by the Roman at 70 CE. Each year, in the 9th of the Hebrew month Av, Jews mourn the destruction of those temples. They pray and fast.)
The Talmud asks and answers:
Why was the second Temple destroyed?
Because gratuitous hatred was rampant in society.
The Talmud wants to warn us from hatred, that can lead to catastrophes.