The Varangian Guard was the Byzantine Emperor's private army of bodyguards. They were vikings hired from Russia and Scandinavia, and later included large numbers of Anglo-Saxon exiles. The Emperors found them very useful, not just for their skill in battle and fierce reputation, but because as outsiders they could be trusted, unlike their native troops who were frequently involved in plots.

Here is an overview of their history: http://en.wikipedia.org/wiki/Varangian_Guard