Short answers, pulled from the story.
The Dorian invasion was a theory that a people called the Dorians swept into the Peloponnese from central Greece shortly after the Trojan War, destroying Mycenaean civilisation. Proposed as a modern archaeological hypothesis by Karl Otfried Müller in 1824, it gained wide scholarly acceptance through the nineteenth century and into the twentieth. By the 1990s, most scholars had rejected it as a myth.
Karl Otfried Müller published The Dorians in 1824, arguing the Dorians were a northern, Indo-European people who invaded and subjugated the Peloponnese. He characterised them as a 'young race', superior in individual character, political organisation and language. His student Ernst Curtius called them 'northern mountaineers'.
Hans Günther, an early Nazi supporter, published Racial History of the Hellenic and Roman Peoples in 1929, arguing the Dorians belonged to the Nordic race. During the period of Nazi rule from 1933 to 1945, German classicists universally portrayed the Dorians as Aryan conquerors. In February 1941, Adolf Hitler declared his belief that the Aryan race had reached its apogee through its supposed conquest of Greece and Italy.
The decipherment of the Mycenaean Linear B script in 1952 showed that Greek speakers had lived in the Peloponnese since the Bronze Age, removing a foundational assumption of the theory. Archaeological discoveries from the 1960s demonstrated that cultural changes attributed to the Dorians predated the palace destructions or originated in regions not associated with the invasion. By the 1990s, most scholars regarded the Dorian invasion as a myth.
Modern archaeologists attribute the collapse of Mycenaean palatial civilisation to endogenous factors including social conflicts, climate change, technological shifts and the breakdown of the palaces' socio-economic model. Population movements at the end of the Bronze Age appear to have run away from Mycenaean centres rather than toward the southern Peloponnese. The collapse is now understood as part of a broader, multi-causal systems collapse rather than the result of a single invasion.
Sparta used the Dorian invasion as its foundation myth, with the poet Tyrtaeus recording in the seventh century BCE that the Spartans had arrived in Laconia from Erineus in Doris, led by the Heracleidae. In 457 BCE, Sparta invoked Dorian kinship to justify the Tanagra campaign, sending troops to defend Doris against Phocis. During the Peloponnesian War at the end of the fifth century BCE, Sparta cited shared Dorian heritage to consolidate its allies against Athens.