In the course of history, tribes of Scandinavian origin took over Germany, Britain, France, Italy, Spain, North Africa, and Russia. Just with respect to Britain, after the Romans, it was taken over by the Angles and the Saxons (Germanic tribes of Scandinavian origin), then later from them by the Normans (of Scandinavian origin). All tribes or nations historically have had some peak before and after which they were not as dominant, so the argument that at some point a particular tribe or nation was no longer dominant is moot. The dominant times of the Vikings, Goths, etc. were very dominant.