In the third century C.E., many old Germanic tribes merged into large federations, like the Saxons, the Franks, and the Alamans, which were to become important during the Middle Ages. To this rule, the Frisians appear to have been an exception: they are already mentioned in sources that deal with the Early Empire. Their ethnogenesis took place at least two or three centuries before the other tribes originated. They can still be distinguished.
At first sight, the Frisians show a remarkable ethnic continuity, but at the beginning of the twentieth century, Pieter Boeles (1873-1961), one of the founding fathers of Frisian archaeology, had different ideas. He noticed important cultural changes in the fourth and fifth century, which he believed were evidence for colonization by the Saxons (from northwestern Germany) and the Anglians (from Schleswich-Holstein). After they had conquered the Frisians, they continued to Flanders, and from there, they conquered parts of Britain. However, Boeles argued, although these tribes subdued the Frisians, their name remained in use, which is why there are still provinces called Friesland in the Netherlands and Germany.
He was right about the Anglo-Saxon settlement – almost. What appears to have happened, is that the country of the Frisians had become empty, for reasons that we do not fully comprehend. In the fourth, fifth, and sixth century, the Frisians are not mentioned in our sources, even by authors who had reason to write about them. When the Saxons and the Anglians arrived, there was no one to subdue, because there was no one living over there.
The name returns in the sources in the seventh century, in an age when other ancient names also return. For example, Gregorius of Tours describes the Frankish king Clovis as a ‘Sugambrian’ – after a tribe that had been annihilated in the first century. These archaisms were usually ignored, but the name of Frisia was accepted. People in what is now North Holland, Utrecht, and Friesland started to call themselves after the district in which the Merovingian and Carolingian authorities had placed them. The continuity of the ancient nation and the ancient name are only apparent.
J. Bazelmans, ‘The early medieval use of ethnic names from classical antiquity. The case of the Frisians’, in: T. Derks en N. Roymans (red.), Ethnic Constructs in Antiquity. The Role of Power and Tradition (2009) blz. 321-337.