The Sea People.

If you want an explanation of the Sea Peoples, the first place to look is inside the pages of the Iliad.  While the events are supposed to take place in late Mycenaean times, much of the world view is that of the later times of the Dorians and the so-called Sea Peoples. You've got a bunch of Achaean pirates laying seige to Troy and the regions thereabout, and we are pretty sure that some of the Sea Peoples (pirates) were indeed Achaeans.

As for what caused this upheaval, a number of reasons are cited. There really was a HUGE earthquake at the end of Mycenaean times. A change in the climate has also been suggested, but I've never seen any evidence presented to support the idea.

More likely it's a technological change: the full arrival of the iron age, with a corresponding democratization of warfare. This also seems to the time when naval warfare first becomes possible. Think of early Iron Age Vikings using hit and run tactics.

Another very serious factor, and probably the decisive one, was the fall of the Hittite Empire. Some very old trade relationships, extending from Sicily and the Gulf of Venice all the way to the Caucasus were severely disrupted. The palace economies of the Greek mainland and islands were suddenly obsolete; the goods they produced had no markets, and with no markets, they themselves ceased being markets for others; they retracted. In some cases, the royal families would seem to have emigrated to greener pastures, or turned pirate themselves.

And, as always, there were the pressures eminating from the Steppe. About this time, the Phyrgians make themselves known; the Greek records say they came from Thrace and Macedonia. Perhaps it was they who pushed the Dorians south into Attica and the Peloponnese, and maybe they were being pushed by Thracians further to the north and east. In any event, some pretty barbaric Indo-Europeans are known to have entered northwest Anatolia at this time (the proto-Armenians seem to be involved here too).

The Sea People are always considered rather mysterious. In most of the details, yes, they are, but we can make a very good guess as to who they were and where they came from. Many of them were Greeks. Others were non-IE Tyrrhenians (a branch of whom became the Etruscans). Some Semites were involved too (via Lebanon and Syria). This was not one concerted invasion, but iron-armed adventurers, freebooters, Ur-Vikings, taking advantage of a power-vacuum and making the most of a technological revolution. 25 and even 50 oar boats combined with iron weapons were quite potent and would remain so until Athens and then later, Rome imposed their will on the Mediterranean. You need a regular fleet to keep piracy under control; even today, without such a fleet, piracy flourishes; witness its resurgence in southern China.

Mark.