Due to a former business agreement of Louis XV of France and his father from March 1751, who promised to the French king to raise a German battalion of infantry for France when and if needed, the Infantry Regiment "Royal Deux-Ponts" (raised on 19, 1757) of two battalions to the French crown after the outbreak of the Seven Years' War, when it was at first deployed in the Battle of Rossbach. The commander of the regiment at this time was his older brother Christian, and Wilhelm was deputy commander.