The Upper Midwest is largely populated by Germans and Scandinavians, mostly because the climate is similar to their homelands, and because so many of them were farmers and land was free for the homesteading in the latter 19th century.
I'm sure there are other ethnic pockets around the country that can tie their existence to some historical event. Large parts of the Dakotas, for example, are peopled by refugee Germans who had immigrated to South Russia under Catherine and were exempted from military service. Later, during the wars of empire that wracked Europe in the 19th century, the Czars reneged on their agreement and began to conscript these displaced Germans. They fled wholesale and tended to stick together in the New World.
I may be descended from some of these folks?
The Scots-Irish (lowland Scots who immigrated to the US from Northern Ireland) settled thickly in the South. Even Mississippi.