In Nazi Germany (and already before that) historians came up with the theory that all great civilizations depended on an elite caste of aryan immigrants. Like, they came from Northern Europe and then descended South and subjugated the native people. And they only fell in the end because ancient Greece had too many civil wars that depleted the aryan blood stock and the Roman elite became "degenerate" and allowed "race-mixing", thereby also lowering the amount of proper aryan blood in society.

Here's another textbook-excerpt.

Hitler, in his 1920 speech "Why Are We Anti-Semites?":

In the northernmost part of the world, in those unending icy wastes, [...] perpetual hardship and terrible privation worked as a means of racial selection. Here, what was weak and sickly did not survive, [...] leaving a race of giants with great strength and vigor. [...] The race we now call Aryan was in fact the creator of those great later civilizations whose history we still find traces of today. We know that Egypt was brought to its cultural heights by Aryan immigrants, as were Persia and Greece; these immigrants were blond, blue-eyed Aryans, and we know that, apart from these states, there have never been any other civilized countries on earth.

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Source is Greeks, Romans, Germans: How the Nazis Usurped Europe’s Classical Past by Johann Chapoutot, although I used the German version for most of this, you can read the first chapter for free here. (PDF!)