This required that the unmarried Marburger heir and the Landgraves of Hessen-Darmstadt should fall, but Amalie Elisabeth had other ideas and led Hesse-Kessel in 1645 into the "Hessenkrieg", ruling as Landgräfin on her son's behalf.

Domination over the Marburger territories went over to the Landgrave of Hesse-Kassel after the accord was dissolved and a new agreement was reached.

William VI succeeded in what his ancestors had tried to do in vain since 1604, that is, to end the Hesse-Marburg landgraviate, and to annex the Marburger lands to Hesse-Kassel.

To finally resolve the quarrel with the landgraves of Hesse-Darmstadt, Wilhelm delivered to George II the territory around Gießen, along with Ämtern by Biedenkopf.

Control of his Landgraviate went to his eldest son William VII, though - not yet of age - he remained under the guardianship of his mother Hedwig Sophie of Brandenburg until his early death in 1670.