Frederick as Crown Prince 1739
Frederick found an ally in his sister, Wilhelmina, with whom he remained close for life. At age 16, Frederick had formed an attachment to the king's 13-year-old page, Peter Karl Christoph Keith. Wilhelmina recorded that the two "soon became inseparable. Keith was intelligent, but without education. He served my brother from feelings of real devotion, and kept him informed of all the king's actions."
When he was 18, Frederick plotted to flee to England with Hans Hermann von Katte and other junior army officers. While the royal retinue was near Mannheim in the Electorate of the Palatinate, Robert Keith, Peter's brother, had an attack of conscience when the conspirators were preparing to escape and begged Frederick William for forgiveness on 5 August 1730; Frederick and Katte were subsequently arrested and imprisoned in Küstrin. Because they were army officers who had tried to flee Prussia for Great Britain, Frederick William leveled an accusation of treason against the pair. The king threatened the crown prince with the death penalty, then considered forcing Frederick to renounce the succession in favour of his brother, Augustus William, although either option would have been difficult to justify to the Imperial Diet of the Holy Roman Empire. The king forced Frederick to watch the decapitation of his confidant Katte at Küstrin on 6 November, leaving the crown prince to faint away and suffer hallucinations for the following two days.
Frederick was granted a royal pardon and released from his cell on 18 November, although he remained stripped of his military rank. Instead of returning to Berlin, however, he was forced to remain in Küstrin and began rigorous schooling in statecraft and administration for the War and Estates Departments on 20 November. Tensions eased slightly when Frederick William visited Küstrin a year later, and Frederick was allowed to visit Berlin on the occasion of his sister Wilhelmina's marriage to Margrave Frederick of Bayreuth on 20 November 1731. The crown prince returned to Berlin after finally being released from his tutelage at Küstrin on 26 February 1732.
Frederick William considered marrying Frederick to Elisabeth of Mecklenburg-Schwerin, the niece of Empress Anna of Russia, but this plan was ardently opposed by Prince Eugene of Savoy. Frederick himself proposed marrying Maria Theresa of Austria in return for renouncing the succession. Instead, Eugene persuaded Frederick William, through Seckendorff, that the crown prince marry Elisabeth Christine of Brunswick-Bevern, a Protestant relative of the Austrian Habsburgs. Although Frederick wrote to his sister that, "There can be neither love nor friendship between us," and he considered suicide, he went along with the wedding on 12…