The ancient Frisii enter recorded history in the Roman account of Drusus's 12 BC war against the Rhine Germans and the Chauci. They occasionally appear in the accounts of Roman wars against the Germanic tribes of the region, up to and including the Revolt of the Batavi around 70 AD. Frisian mercenaries were hired to assist the Roman invasion of Britain in the capacity of cavalry. They are not mentioned again until 296, when they were deported into Roman territory as laeti. The discovery of a type of earthenware unique to 4th century Frisia, called terp Tritzum, shows that an unknown number of them were resettled in Flanders and Kent, probably as laeti under Roman coercion. From the 3rd through the 5th centuries Frisia suffered marine transgressions that made most of the land uninhabitable, aggravated by a change to a cooler and wetter climate. Whatever population may have remained dropped dramatically, and the coastal lands remained largely unpopulated for the next two centuries. When conditions improved, Frisia received an influx of new settlers, mostly Angles and Saxons. These people would eventually be referred to as 'Frisians', though they were not necessarily descended from the ancient Frisii. It is these 'new Frisians' who are largely the ancestors of the medieval and modern Frisians. By the end of the 6th century, Frisian territory had expanded westward to the North Sea coast and, in the 7th century, southward down to Dorestad. This farthest extent of Frisian territory is sometimes referred to as Frisia Magna. Early Frisia was ruled by a High King, with the earliest reference to a 'Frisian King' being dated 678. In the early 8th century the Frisian nobles came into increasing conflict with the Franks to their south, resulting in a series of wars in which the Frankish Empire eventually subjugated Frisia in 734. These wars benefited attempts by Anglo-Irish missionaries to convert the Frisian populace to Christianity, in which Saint Willibrord largely succeeded. Some time after the death of Charlemagne, the Frisian territories were in theory under the control of the Count of Holland, but in practice the Hollandic counts, starting with Count Arnulf in 993, were unable to assert themselves as the sovereign lords of Frisia. The resulting stalemate resulted in a period of time called the 'Frisian freedom', a period in which feudalism and serfdom did not exist, and in which the Frisian lands only owed their allegiance to the Holy Roman Emperor. During the 13th century, however, the counts of Holland became increasingly powerful and, starting in 1272, sought to reassert themselves as rightful lords of the Frisian lands in a series of wars, which ended in 1422 with the Hollandic conquest of Western Frisia and with the establishment of a more powerful noble class in Central and Eastern Frisia. In 1524, Frisia became part of the Seventeen Provinces and in 1568 joined the Dutch revolt against Philip II, king of Spain, heir of the Burgundian territories; Central Frisia has remained a part of the Netherlands ever since. The eastern periphery of Frisia would become part of various German states and Denmark. An old tradition existed in the region of exploitation of peatlands.
Though impossible to know exact numbers and migration patterns, research has indicated that many Frisians were part of the wave of ethnic groups to colonise areas of present day England alongside the Angles, Saxons and Jutes, starting from around the fifth century when Frisians arrived along the coastline of Kent. Studies have found the DNA of people tested in Central England to be "indistinguishable" from that of Frisians. Frisians principally settled in modern-day Kent, East Anglia, the East Midlands, North East England, and Yorkshire. Across these areas, evidence of their settlement includes place names of Frisian origin, such as Frizinghall in Bradford and Frieston in Lincolnshire. Similarities in dialect between Great Yarmouth and Friesland have been noted, originating from trade between these areas during the Middle Ages. Frisians are also known to have founded the Freston area of Ipswich. In Scotland, historians have noted that colonies of Angles and Frisians settled as far north as the River Forth. This corresponds to those areas of Scotland which historically constituted part of Northumbria.
Language
As both the Anglo-Saxons of England and the early Frisians were formed from similar tribal confederacies, their respective languages were very similar, together forming the Anglo-Frisian family. Old Frisian is the most closely attested language to Old English and the modern Frisian dialects are in turn the closest related languages to contemporary English that do not themselves derive from Old English. The Frisian language group is divided into three mutually unintelligible languages:
West Frisian, spoken in the Dutch province of Friesland
Saterland Frisian, spoken in the German municipality of Saterland just south of East Frisia
North Frisian, spoken in the German region of North Frisia on the west coast of Jutland.
Of these three languages both Saterland Frisian and North Frisian are endangered. West Frisian is spoken by around 350,000 native speakers in Friesland, and as many as 470,000 when including speakers in neighbouring Groningen province. West Frisian is not listed as threatened, although research published by Radboud University in 2016 has challenged that assumption.
Identity
Today there exists a tripartite division of the Frisians, into North Frisians, East Frisians and West Frisians, caused by Frisia's constant loss of territory in the Middle Ages. The West Frisians, in general, do not see themselves as part of a larger group of Frisians, and, according to a 1970 poll, identify themselves more with the Dutch than with the East or North Frisians. Therefore, the term 'Frisian', when applied to the speakers of all three Frisian languages, is a linguistic, ethnic and/or cultural concept, not a political one.