The Burial of the Dead of Beitar
Hadrian left the slain of Beitar unburied for years; when burial was finally allowed on Tu B'Av, the bodies had miraculously not decayed.
Rav Masneh stated that Tu B’Av was the day that the Roman occupiers permitted burial of the victims of the massacre at Beitar. Beitar was not ravaged until 52 years after the destruction of the second Beis HaMikdash. The residents of Beitar did not suffer from the malady of baseless hatred; they were therefore spared the fate of Yerushalayim. The Midrash therefore asks: What sin precipitated their eventual downfall? It was that they celebrated when the people of Yerushalayim were suffering.
However, this was not baseless hatred. The Midrash notes that the people of Yerushalayim managed to steal the titles to their lands. Some residents of Beitar were left penniless because of this fraud, so they indeed had just cause to be incensed. Still, one should not rejoice at the downfall of his enemy. This was the sin that sealed their tragic fate.
The Romans killed thousands upon thousands of Jews from Beitar. Hadrian, the wicked Roman emperor, refused to allows the corpses to be buried. The next emperor, after receiving sufficient compensation, finally acquiesced to the burial (Avudraham). Miraculously, the bodies had not decomposed, despite exposure to the elements for years. After the terrible tragedies, some Jews questioned whether Hashem still loved them. This open miracle was a sign from Hashem that He still loves us and is with us even in exile. To experience such a clear miracle after the Jewish nation was already exiled was indeed cause for celebration.