Along with mass migrations

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Grand Solar Minimums though history have caused civilisations to fall and others to rise

J.H. Walker

You could argue that the westward migration of the Hun, Angles, Saxons and Franks into Eastern France, and the Normans (Norwegians) into Northwestern France were due to cold-climate changes stemming from the Late Antique Little Ice Age (LALIA) period from 585 to 630AD and other Grand Solar Minimums (GSMs) since.

That LALIA cold-climate-change period started the movement of many of the then Russian steppes horse tribes westward over a three-hundred year migration on foot and horse.

It also created the conditions for the rise of Islam and ability of Middle Eastern grasslands to support horses for their light cavalry tactics.

The descendants of the European migrating tribes formed new nations and recommenced the move westward via sea during the next series of GSMs (which form the Little Ice Age) to what they hoped was an empty continent, but was already occupied by similar human migrants moving eastward via the glaciation land bridges.