King George II, and they became the parents of four children. In 1749, after increasing disillusionment with Calvinism, he became a secret convert to Roman Catholicism. His conversion was purely personal and when, in 1760, he succeeded as ruler, Hessen remained a leading 
Calvinistic state. 
In 1755 he separated formally from his wife and, in 1756, entered Prussian military service, rising after considerable experience to the rank of field marshal. He hired out mercenary troops, notoriously to Britain for use in the American War of Independence. A tolerant and humane ruler, he was influenced by enlightened ideas and carried through important measures of modernization, 
especially in education and welfare. He was responsible for the reconstruction of his state after it had suffered serious devastation in the Seven Years War. 
He contributed to the beautification of his capital, Kassel, commissioning major new buildings. He was unusual in that he co-operated throughout with representatives of his subjects in the 
parliamentary estates. Possibly because of this, he was the most successful eighteenth-century ruler of Hessen-Kassel. 
In 1772 his wife, from whom he had been separated for sixteen years, died and a year later he married Philippine von Brancenburg-Schwedt, but this marriage remained childless. On 31 October 1785 he died at Castle Weissenstein.
 

Source: Leo van de Pas