1514-1517
In this period, the Saxon feud was fought, in which George, Duke of Saxony, allied with the Habsburg, tried to subdue Frisia, the Ommelanden and Groningen. The Frisians under leadership of Jancko Douwama resisted and were supported by Edzard I, Count of East Frisia and the inimitable Charles, Duke of Guelders. George of Saxony failed to conquer Groningen and was pushed back. When he held only a handful of cities (Leeuwarden, Harlingen en Franeker), he sold his rights to Charles of Habsburg in May 1515.
In the meantime, the Frisian rebels, led by the legendary Pier Gerlofs Donia and Wijerd Jelckama won many battles at sea and raided through Holland at the head of the Arumer Zwarte Hoop.
Because Charles of Habsburg needed to consolidate the Spanish throne and manoeuver to become Holy Roman Emperor in Germany, he concluded peace with Edzard I, recognizing him as Count of East-Frisia and with Charles, Duke of Guelders, leaving him in control of most of Frisia, the Ommelanden and Groningen. Charles of Guelders consequently conquered Overijssel in 1521-1522, with the support of Zwolle.
Read more about this topic: Guelderian Wars