One of the most able of his line, the sitter was the son-in-law of King George II of England - although his wife, Princess Mary, separated from him on his conversion to Catholicism, retiring to Hanau with her son. The Landgrave meanwhile took a former mistress of the Duc de Bouillon, but set up no pretensions to fidelity, and is said to have had more than one hundred children. Frederick's role on the world stage was greatly magnified by the onset of the American Revolution. The Landgraves of Hesse-Cassel had on more than one previous occasion hired their troops out to supplement the state's income and Frederick's troops, drilled on the Prussian system, and recruited in a measure among his own subjects by conscription, were good soldiers. The Landgrave negotiated with King George III for twelve thousand of these to fight on the British side in the war for over £3,000,000, forming the largest contingent of continental troops hired by the King, and one that would lend its name to the generic term for such forces.
Whatever opinion may be held as to this display of realpolitik, it is certain that Frederick spent the money well, doing much for the development of the economic and intellectual improvement of the country. Possessing a liberal and humanitarian streak, he was able to diminish the taxes of his remaining subjects, found schools and museums, be a keen patron of the arts, maintaining a French theatre and opera, with a French corps de ballet at Cassel, and generally take pains with the internal affairs of his country, still leaving a full treasury at his death.