During the late 3rd millennium BC there began a series of invasions by tribes from the north who spoke an Indo-European language. Evidence exists that the northerners originally inhabited the basin of the Danube River in southeast Europe. The most prominent of the early invaders, who were to be called the Achaeans, had, in all probability, been forced to migrate by other invaders. They overran southern Greece and established themselves on the Peloponnesus, According to some scholars, a second tribe, the Ionians, settled chiefly in Attica, east-central Greece, and the Cyclades, where they were assimilated to a great degree with the Helladic people. The Aeolians, a third, rather vaguely defined tribe, originally settled in Thessaly.

Gradually, in the last period (circa 1500-1200 BC) of Bronze Age Greece, the mainland absorbed the civilization of Crete. By 1400 BC the Achaeans were in possession of the island itself, and soon afterward they became dominant on the mainland, notably in the region around Mycenae. Although this city has given its name to the Achaean ascendancy because of the extensive archaeological investigations of its ruins, other city-kingdoms were of great, if not equal, importance. The Trojan War, described by Homer in the Iliad, began about, or shortly after, 1200 BC and was probably one of a series of wars waged during the 13th and 12th centuries BC. It may have been connected with the last and most important of the invasions from the north, which occurred at a similar time and brought the Iron Age to Greece. The Dorians left their mountainous home in Epirus and pushed their way down to the Peloponnesus and Crete, using iron weapons to conquer or expel the previous inhabitants of those regions. The invading Dorians overthrew the Achaean kings and settled, principally, in the southern and eastern part of the peninsula. Sparta and Corinth became the chief Dorian cities. Many of the Achaeans took refuge in northern Peloponnesus, a district afterward called Achaea. Others resisted the Dorians bitterly, and after being subjugated were made serfs and called helots. Refugees from the Peloponnesus fled to their kin in Attica and the island of Euboea, but they later migrated, as did the Aeolians, to the coast of Asia Minor.

In the centuries after 1200 BC the increased colonization of the Asia Minor coast, first by refugees from the Dorians and then by the Dorians themselves, made the area a political and cultural part of Greece. Three great confederacies were established by each of the Greek ethnic divisions. The northern part of the coast of Asia Minor and the island of Lesbos constituted the Aeolian confederacy. The Ionian confederacy occupied the middle district, called Ionia, and the islands of Chios and Samos. A Doric confederacy was established in the south and on the islands of Rhodes and Kos.

Several centuries later (750-550 BC) a rapid population increase and a consequent shortage of food, the rise of trade and industry, and other conditions led to another great colonizing movement. Colonies were established in places as widely separated as the eastern coast of the Black Sea and what is now Marseille, France, and included settlements in Sicily and the southern part of the Italian peninsula. The latter was so thickly inhabited by Greeks that the area became known as Magna Graecia (Latin, "Greater Greece").