In June 1774 he entered the Prussian Army as an oberst in the Kürassierregiment Lölhöffel, rising to a commander in the same unit in December 1776. He fought with it in the War of the Bavarian Succession. In 1780 he took over the 2nd Dragoon Regiment (Krockow). Frederick married Duchess Augusta of Brunswick-Wolfenbüttel (sixteen years old and thus ten years his junior) on 15 October 1780 at Braunschweig. She was the eldest daughter of Charles William Ferdinand, Duke of Brunswick-Lüneburg and Princess Augusta of Great Britain and thus a niece of George III of Great Britain and sister to Caroline of Brunswick, the future wife of George IV. The marriage was not a happy one - even during her first pregnancy in 1781 she wished to separate but was persuaded to stay with Frederick by her father. Though they had four children, Frederick was rumored to be bisexual, with a of young noblemen.