Frisian history

Frisia has changed dramatically over time, both through floods and through a change in identity. It is part of the Nordwestblock which is a hypothetical historic region linked by language and culture.
Roman times.
The Frisii began settling in Frisia around 500 BC, though recent farming settlements have been dug up which date back to 2300BC. 
(source : https://frieschdagblad.nl/2020/8/5/het-raadsel-van-een-2300-jaar-oude-sloot-bij-leeuwarden) 
Which may contradict that the Frisians have settled only in 500BC.
"2300-jaar-oude-sloot" means: a ditch 2300 years old, so dug about 300BC
According to Pliny the Younger, in Roman times, the Frisians (or, as it may be, their close neighbours, the Chauci) lived on terps, man-made hills. According to other sources, the Frisians lived along a broader expanse of the North Sea (or "Frisian Sea") coast.
Frisia at this time comprised the present-day provinces of Friesland and North Holland.
Frisians appear to have been among the Germanic groups who invaded Britain during the so-called Migration period ("Völkerwanderung"), as Angles and Saxons travelled from their home base through Frisian territory in what is now northern Germany and central Netherlands.
Kingdom of Frisia.
The 8th-century historian Bede also used the term 'Frisians' for Franks in the south of Frisia. East Anglian sources called the inhabitants of 'Frisia' Warnii instead of Frisians. In the 7th and 8th centuries, the Frankish chronologies mention this area as the kingdom of the Frisians. However, these were probably not the Frisians of Roman times. This kingdom comprised the coastal provinces of the Netherlands and the German North Sea coast. During this time, the Frisian language was spoken along the entire southern North Sea coast and, today, this region is sometimes referred to as Greater Frisia or "Frisia Magna". The 7th-century Frisian realm (650-734) under the kings Aldegisel and Redbad, had its centre of power in the city Utrecht. Its end came in 734 at the Battle of the Boarn, when the Frisians were defeated by the Franks, who then conquered the western part up to the Lauwers. They conquered the area east of the Lauwers in 785, when Charlemagne defeated Widukind. This "Frisia Magna" was partly occupied by Vikings in the 840s, until they were expelled between 885 and 920. It has also been suggested that the Vikings did not conquer Frisia, but settled in certain parts (such as the island of Wieringen), where they built simple forts and cooperated and traded with the native Frisians. One of their leaders was Rorik of Dorestad.
Loss of territory.
Frisians made polders in West Friesland, which became more and more separated from Friesland because of floods. The western part of Frisia became the county of Holland in 1101, after a few centuries of a diverging history than the other parts. Frisia began to identify itself as a country with free folk in the Middle Ages. The bishopric of Utrecht no longer belonged to Frisia. There were many floods in the 11th and 12th centuries, which led to the deaths of many and eventually formed the Zuider Zee. The largest flood occurred in 1322, in which many hundreds of people drowned.
Opstalboom League.
The free Frisians and the city of Groningen founded the Opstalboom League to counter feudalism. The league consisted of modern Friesland, Groningen, East Frisia and the German North Sea coast, and parts of the Danish North Sea coast (Schleswig). But the Opstalboom league did not consist only of Frisians, as the area of Zevenwouden and the city of Groningen were Saxon. Some Frisians lived under the rule of the counts of Holland in West Friesland. The Opstalboom League was short-lived; it collapsed after a few years because of continual internal strife.
15th century.
The 15th century saw the end of the free Frisians. The city of Groningen started to dominate Groningen. A petty nobleman in East Frisia managed to defeat the other petty noblemen and became count of East Frisia. The Archbishop of Bremen-Hamburg and the king of Denmark conquered large areas of Frisia. Only Friesland remained for the Frisian Freedom. Friesland was conquered in the 1490s by Duke Albert of Saxony-Meissen. Later, the giant Pier Gerlofs Donia (Grutte Pier) would fight for his country's freedom. He had many successes, but ultimately failed to secure Frisia's independence. He died a poor farmer in 1520. In 1519 already, his lieutenant Wijard Jelckama took over the command of Donia's armies. He was less successful, yet did have minor victories. When Jelckama eventually got caught in 1523, his armies were not even one third the size they were when he took over four years earlier. He was then tried, found guilty of treason, and decapitated.