Having come to rule in unfavorable circumstances and in the midst of the Thirty Years' War, he continued to suffer losses of territory and wealth.

However, when he converted to Calvinism the next year, he entered into a protracted legal dispute with his Lutheran cousin Landgrave Louis V of Hesse-Darmstadt.

The Aulic Council decided in favour of Louis, and on 17 March 1627, after losing much of the territory to the Darmstadt line and leaving the family in financial ruin, Maurice finally abdicated in favor of his son.

In the Thirty Years' War, he allied with his relative King Gustavus Adolphus of Sweden, whose army disembarked on 6 July 1630 in Pomerania.

William's expectations proved to be false, when Gustavus Adolphus only seized the Rüsselsheim fortress and left Upper Hesse to Landgrave Georg.