The complete change in the map of Greece at the close of the Achæan period and the origin of the ethnographic system with which the history of Hellenic times begins, were always referred by Greek tradition to a last wandering of north Grecian tribes. The customary chronology places the beginning of this shifting at 1133 or 1124 B.C., i.e., less than three generations after the so-called conquest of Troy. Recent chronological investigations, however, have made it seem probable that a period at least a hundred years later should be chosen.
The first impulse was probably given by new movements of tribes in the north. The advance of the Illyrians caused the Thessalians, a part of the Epirot tribe of the Thesproti, to withdraw across Pindus into the valley of the Peneus, which was afterwards called Thessaly. While the preservation of the Greek character in Epirus was henceforth left to the brave Molossi, the Thessalians east of Pindus fell upon the settled Greeks of the lowlands and destroyed their states. The proudest and most vigorous elements of the old population that survived the war, determined to emigrate and found a new home. Thus, the Arnæ migrated to middle Greece, destroyed the old states of Thebes and Orchomenus in the basin of the Copaïs and united this whole district, which henceforth appears in history as Bœotia, under their rule.
While the Thessalians were making preparations to subjugate the warlike tribes of the highlands about the valley of the Peneus, one of these mountain races, the Dorians, carried the mighty movement on to the extreme south of the Peloponnesus. Within twenty years, according to tradition, they had crossed the narrow strait of Rhium and begun the conquest of the Peloponnesus. They ascended the valley of the Alpheus into southern Arcadia. From here one body of them descended into the Messenian valley of the Pamisus and overwhelmed the old kingdom of the Melidæ of Pylos. The other branch invaded the principal districts of the Achæans in the east and southeast of the Peloponnesus. In open battle the rude Dorian foot-soldiers easily defeated the Achæan knights. But they could not destroy the colossal walls of the Achæan fortresses or cities, and were themselves finally forced to build fortifications from which they could watch or invest the Achæan strongholds until the opportunity was presented of storming them or forcing their capitulation. It was in such a fortified camp that the Dorian capital Sparta had its origin.
It was probably the tenacious resistance of the Achæans in Laconia that determined a large body of the Dorians to leave that district and turn to the east, where they completely subjugated Argolis and made Argos the centre of Dorian power in the eastern part of the Peloponnesus.
At the close of the Achæan period Attica was the canton which appeared to have the most settled and uniform structure. It now became a favourite refuge of migrant Greeks of many different tribes. This movement seems to have strengthened little Attica in a considerable degree, for tradition ascribes to these immigrants the successful resistance that Attica was able to make when the hordes of the conquerors finally approached her borders. But Attica was far too small and unproductive to retain the mass of fugitives as permanent settlers. So the movement was finally turned towards the islands of the Ægean and the coast of Asia Minor. According to tradition there had already been an Archæan (or Æolian) migration to Lesbos and Tenedos, from whence the Mysian coast and Troas were later colonised.
The most important Ionian colonies in the east were in the Cyclades, at Miletus, and at Ephesus. As their power continued to grow, the Ionians gradually Hellenised a broad strip of coast and in the river valleys pushed out a considerable distance to the eastward.
The Dorians also followed the movement of the other Greeks to the islands and to Asia. Their most important occupations were Crete, Rhodes, and a small portion of the southern coast of Caria, including the cities of Cnidus and Halicarnassus.
By the first half of the eighth century B.C., the Greek world had acquired the aspect which it retained for several centuries. The nation had greatly increased its territory by colonisation. But the district now called Thessaly was in possession of a race that showed little capacity to develop beyond a vigorous and pleasure-loving feudalism; and the Greeks of Epirus and the valley of the Achelous had been for several centuries shut out from the evolution into Hellenism. So apart from the newly risen power of the Bœotians, the future of Greece rested upon the two races that had been but little named in the Achæan period. The Dorians had become a great people. Argos had at first been the leading power of the Peloponnesus, both in religion and in politics. The Doric canton in the valley of the Upper Eurotas had made but slow and difficult progress, until, at the close of the ninth and beginning of the eighth century, that remarkable military and political consolidation was completed which is connected with the name of Lycurgus. This was the starting-point of a growth of Spartan power in consequence of which before the end of the eighth century the balance of Doric power was to pass from Argos to the south of the Peloponnesus.
Among the Ionians the Asiatic branch long remained the more important. The Ionian Greeks of the Ægean and of the Lydio-Carian coast, through their direct contact with the Orient, introduced to the Greek world new elements of culture of a varied character. Of a friendly and adaptable nature, they were specially fitted to be the traders and mariners of Greek nationality. Politically they became pre-eminently the democratic element of the nation, although there were powerful aristocratic groups among them. But with them the tendency appears stronger than among the other Greeks to allow full scope to personality, individual right, individual liberty, and individual activity beside, and even in opposition to the common interest.