quality:corruption, structural:summarization
Relationship with Frederick II

His eldest surviving son was Frederick II (Fritz), born in 1712. Frederick William wanted him to become a fine soldier. As a small child, Fritz was awakened each morning by the firing of a cannon. At the age of 6, he was given his own regiment of children to drill as cadets, and a year later, he was given a miniature arsenal.

The love and affection Frederick William had for his heir initially was soon soured due to their increasingly different personalities. Frederick William ordered Fritz to undergo a minimal education, live a simple Protestant lifestyle, and focus on the Army and statesmanship as he had. However, the intellectual Fritz was more interested in music, books and French culture, which were forbidden by his father as decadent and unmanly. As Fritz's defiance for his father's rules increased, Frederick William would frequently beat or humiliate Fritz (he preferred his younger sibling Augustus William). Fritz was beaten for being thrown off a bolting horse and wearing gloves in cold weather.

At age 16, Frederick seems to have embarked upon a youthful affair with Peter Karl Christoph von Keith, a 17-year-old page of his father. Rumors of the liaison spread in the court, and the "intimacy" between the two boys provoked the comments of his sister, Wilhelmine, who wrote, "Though I had noticed that he was on more familiar terms with this page than was proper in his position, I did not know how intimate the friendship was." Rumors finally reached King Frederick William, who cultivated an ideal of ultramasculinity in his court, and derided his son's supposedly effeminate tendencies. As a result, Keith was dismissed from his service to the king and sent away to a regiment by the Dutch border, while Frederick was sent to the king's hunting lodge at Königs Wusterhausen in order to "repent of his sin".

After the prince attempted to flee to England with his tutor, Hans Hermann von Katte, the enraged king had Katte beheaded before the eyes of the prince, who himself was court-martialled. The king may have thought that Frederick's relationship with Katte was also romantic, a suspicion that may have played a role in Katte receiving a death sentence. In any case, the court declared itself not competent in the case of the crown prince. Whether it was the king's intention to have his son executed as well (as Voltaire claims) is not clear. However, the Holy Roman Emperor Charles VI intervened, claiming that a prince could only be tried by the Imperial Diet of the Holy Roman Empire itself. Frederick was imprisoned in the Fortress of Küstrin from 2 September to 19 November 1731 and exiled from court until February 1732, during which time he was rigorously schooled in matters of state. After achieving a measure of reconciliation, Frederick William had his son married to Princess Elisabeth Christine of Brunswick-Wolfenbüttel-Bevern, whom Frederick despised, but then grudgingly allowed him to indulge in his musical and literary interests again. He also gifted him a stud farm in East Prussia, and Rheinsberg Palace. By the time of Frederick William's death in 1740, he and Frederick were on at least reasonable terms with each other.

Although the relationship between Frederick William and Frederick was clearly hostile, Frederick wrote in his memoirs that his father "penetrated and understood great objectives and knew the best interests of his country better than any minister or general."