My knowledge is that previous to and over the existence of the US, Germans constitued the largest part of the immigrants from anywhere. It is just that their immigaration was a continuous one, without too many ups and downs and that they assimilated quickly and quietly, so there was never a real perception of them as an ethnic group.
I remember how almost anyone in the Nixon administration in the 70ies - not just the heavy-accented guy from Nürnberg - had German origin last names.