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 c. 296, when they were deported into Roman territory as laeti (i.e., Roman-era serfs; see Binchester Roman Fort and Cuneus Frisionum). 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 (which had begun with Saint Boniface) 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 (as well as central or judicial administration) 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 (with a series of lengthy interruptions) 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 (later Germany) and Denmark. An old tradition existed in the region of exploitation of peatlands.