King Henry IV of France had been friendly with La Trémoille when he was King of Navarre, but kept him in a subordinate position once he became King of France, preferring La Trémoille's cousin, Henry de La Tour d'Auvergne, the Viscount of Turenne. In 1587, La Trémoille converted to Protestantism. He fought for Henry IV at the battle of Coutras and also at Ivry, and was rewarded by elevation to the peerage, as Duke of Thouars, in 1595. This new title, however, made La Trémoille lose more money than it earned him. In 1598, Turenne proposed to his sister-in-law Charlotte-Brabantine to marry La Trémoille. Thanks to her relations with the houses of Orange and Bouillon, Charlotte-Brabantine played an important part in the French Protestant diplomacy. They had four children: Henry (1598–1674); Charlotte (1599–1664), who married James Stanley, Earl of Derby; Élisabeth (1601–1604) and Frédéric (1602–1642) comte de Laval.