linguistic:paraphrasing, structural:expansion
Nobuyasu Incident

In 1579, Lady Tsukiyama, Ieyasu's wife, and his heir Nobuyasu were accused by Nobunaga of conspiring with Takeda Katsuyori to assassinate Nobunaga, whose daughter Tokuhime was married to Nobuyasu. For this reason, Ieyasu ordered his wife to be executed and forced his son to commit seppuku.

There are various theories regarding this incident. According to the "Mikawa Monogatari'", which was written by Ōkubo Tadachika, Tokuhime (wife of Nobuyasu), who was not on good terms with her mother-in-law Tsukiyama-dono, wrote in a letter to her father, Nobunaga, that her mother-in-law and her husband were secretly conspiring with Takeda Katsuyori.

However, this hypothesis was considered obsolete by various historians in the modern era. According to Katsuhiro Taniguchi, the more plausible theory was there was friction within the house of Tokugawa clan between two factions with conflicting ideals, as proposed by the Japanese writer Tenkyu Goro. One faction was active on the front lines and had many opportunities to advance their careers, dubbed the "Hamamatsu Castle Faction." The other faction was the "Okazaki Castle Faction," which consisted of Tokugawa vassals responsible for logistical support due to past injuries and other factors that caused them to play a lesser role in the current politics of Tokugawa clan. According to this theory, the conflict between these two factions eventually led to a conflict between Ieyasu and his son Nobuyasu, who represented the Okazaki faction, finally ending with Nobuyasu's death in prison. Nobuyasu planned to exile Ieyasu with the help of the Okazaki Castle faction. Before and after his son's execution, Ieyasu punished or executed many of those who worked at Okazaki Castle, and some escaped from the castle. Taniguchi theorized that Tsukiyama also participated in the coup d'état that was going on in Okazaki Castle. Furthermore, Sakai Tadatsugu, the most prominent general of Ieyasu, also may have played a role in confirming Oda Nobunaga's suspicion of the alleged betrayal against the Oda clan being planned by Nobuyasu Tsukiyama. Nobunaga thus may have concluded that if a high-ranking Fudai daimyō such as Tadatsugu confirmed the accusation against Lady Tsukiyama, then her treason must be true.

It was said in another theory that Tadatsugu was actually conspiring with the mother of Ieyasu, Odai no Kata, to get rid of Lady Tsukiyama. Arthur Lindsay Sadler theorized this was a deliberate act of spite from Tadatsugu due to many senior Tokugawa clan generals' dislike of Nobuyasu.

In the same year Ieyasu named his third son, Tokugawa Hidetada, as his heir since his second son had been adopted by Toyotomi Hideyoshi, who would later become an extremely powerful daimyo.