| The invasion of the Doric tribes to Greece at the end of the 2nd millennium BCE (starting at around 1200 BCE), put an end to the Mycenean culture, that was predominant in Greece until then. It is unclear whether the invasion of the Dorians was the reason for the collapse of the Mycenean culture, or whether its decline and fall simply enabled the invasion, but it is of little importance. The fact is that during a period of half a century the entire cultural and national structure of Greece was altered completely.
According to our knowledge {that is based principally on the distribution of dialects in archaeic and classical Greece (cf. Ionic dialect, Doric dialect, Aeolic dialect, Arcado-Cyprian dialect)}, and, to a certain extent, on archaeological findings), it seems that the Dorians (that until then resided in the areas of west Macedonia), passed first through the region known as Doris, south of Thessaly (and from that area they gained their name), and turned directly to the bay of Corinth, which they crossed in ships and took control of the Isthmus, the region of the Megaris (the land of Megara), and the eastern and southern parts of the Peloponnese.
This was only a part of the invasion, for almost at the same time and in the following decades, from the same areas several groups set out to conquer Boeotia and Thessaly. Groups of Aetolics who went to the west and took over Locris and Phocis and later also Achaea and Elis.
The human waves of invaders, that continued for about 50 years, changed completely the human and linguistic map of Greece. Only two regions remained unchanged: Attica, which was settled densely (and probably quite powerful), and was located remotely from the principal invasion routes, so that the Dorians, who looked for the the wealthy and important regions of the Isthmus and the Peloponnese, skipped it entirely, and Arcadia, that was protected by its mountains. The Arcadians held out and drove off the invaders from all directions, but they lost their only outlet to the sea, in the west.
The result of the invasion was the complete elimination of the Ionic culture in most of mainland Greece (with the notable exception, as I mentioned previously, of Athens in Attica). The Ionians (as well as many of the Aeolic tribes) left Greece through Athens, to the Aegean islands and Asia Minor, where they founded the cities of Ionia and Aeolis.
The fate of those who remained behind was diverse: in Messenia, Thessaly and Boeotia the invaders mixed with the local population almost entirely. In most of the other lands they were enslaved and all traces of their former national identities were slowly erased as they integrated with their conquerers. In Laconia the situation was quite different, as the local population was divided into two classes: the Perioikoi and the Heilotes, both were denied many rights, but as the perioikoi had their personal freedom at least, the heilotes became the slaves of the community, and the enemies of the state. | 
