In 1517, disagreements between the Duke of Guelders and Jancko concerning the planning of the governing board of Friesland, and Guelders refusing to recognise Douwama as hereditary-lord eventually led to Jancko changing allegiances. In 1521 Jancko swore an oath of allegiance to the Holy Roman Emperor Charles V of Habsburg, the arch-enemy of the Duke of Guelders.
A rift developed in 1517 between the Duke of Guelders and Jancko over the formation of Friesland's governing council, which led to Guelders' denial of Douwama's hereditary status. This prompted Jancko to defect to the opposing side. Two years later, in 1521, Jancko swore loyalty to Charles V of Habsburg, the arch-nemesis of the Duke of Guelders.