Some time around 1200BCE, almost every city in the eastern mediterranean from Troy to Gaza, as well as on the Greek mainland and Cyprus, was violently destroyed or abandoned. Both the Mycenean (early Greeks) and Hittite empires collapsed completely, along with an entire branch of the Egyptian empire.
Evidence points towards numerous attacks by "sea peoples" but we have no idea where they originated (other than, yknow, the sea) or why they suddenly attacked en masse when they did. The destroyed regions were not conquered or re-settled as far as we can tell.
Textual evidence from the time feels almost apocalyptic. From a tablet written by a Hittite king:
My father, behold, the enemy's ships came; my cities were burned, and they did evil things in my country. Does not my father know that all my troops and chariots are in the Land of Hatti, and all my ships are in the Land of Lukka?...Thus, the country is abandoned to itself. May my father know it: the seven ships of the enemy that came here inflicted much damage upon us.