Ah, the Nobuyasu Incident, one of the biggest mysteries of the Tokugawa and Sengoku.

The "evidence"

The early 17th century Mikawa Gofudoki says Lady Tsukiyama sent the following letter to Takeda Katsuyori:

Saburō Nobuyasu is my son, so I will do anything to convince him to ally with the Takeda. I have plans for the two generals Tokugawa and Oda. [We] shall certainly defeat them. If this is done, Ieyasu's old domain should be given to Nobuyasu as is. Also, as for me, please give me to one of your appropriate vassals as his wife. If you agree to these terms, quickly send me your written oath. I too shall quickly carry out [my/the] plans.

The response is as follows:

I have read the letter you had Genkyō deliver me. You said you'll convince your son Lord Saburō to ally with Katsuyori, and you'll with plot kill Nobunaga and Ieyasu, on that we don't even need to mention Ieyasu's realm, from Nobunaga's realm you may pick any province you wish and it'll be added to your realm. Next, Lady Tsukiyama, luckily there's a great samurai called Oyamada Saemon [Nobushige] of Gunnai [eastern Kai] who lost his wife last year and is currently a widower, so you may go be his wife. If you and Nobuyasu agree, we shall first send people to Kai.
If any breaks with the right [above], may he suffer all divine punishments of Brahmā, Śakra, the Four Heavenly Kings, and all the gods of over sixty provinces of Japan large and small, the Gongens of Izu and Hakone, Mishima Daimyōjin, Hachiman Daijingū, and Tenmandaijizaitenjin [Tenjin]. The oath was as stated.
Tenshō 6.XI.xvi [December 14, 1578]
Katsuyori (blood seal)
To Lady Tsukiyama

The complaints Princess Toku sent to Nobunaga, along with the letter from Katsuyori, according to the Mikawa Gofudoki is as follows:

• Lady Tsukiyama is an evil person and said all sorts of bad things about me to Lord Saburō and caused discord.

• Lady Tsukiyama said I was useless for only bearing daughters, and said only sons are important to great generals who go to battle, and advised Lord Saburō to take many concubines, and with that called the daughter of Hinata Lord of Yamato, a vassal of Katsuyori, there-by tried to hook up with Kai
• Lady Tsukiyama has a secret husband called Genkyō, who’s a Chinese doctor from Kai, and sent him to hook up Nobuyasu with Katsuyori, and he sent an oath letter that as allies of Kai once Lord Nobunaga and Lord Ieyasu are killed to give Nobuyasu his father’s lands plus Oda province, and Lady Tsukiyama would be the wife of some samurai called Oyamada.
• Lord Saburō is always barbaric. He killed my lady-in-waitings called Kojijō right in front of me, and on top of that cut her mouth open.
• Lately Lord Saburō likes dances, and when he sees dancers who do not dress well or dance properly, he shoots the dancer to death right then and there
• Lord Saburō went out falconing and hunting and there met one with tonsure, and he blamed that he didn’t get kills today on the monk and he tied the monk’s neck to a horse and had the horse run, killing him.
• In Katsuyori’s letter he says to do anything to hook up [Lord Saburō] and make him an ally, so if you’re not careful he may end up with our enemies as said previously.

The reason for Lady Tsukiyama’s actions, as given in the early 17th century Matsudairaki, which has a summary of what’s in Mikawa Gofudoki, is that her father was (assistant) Justice Minister Sekiguchi (an Imagawa branch family) and was given to Ieyasu as a wife to keep an eye out on him, her and their children were hostages in Sunpu when Ieyasu switched sides after Okehazama, which also costed her parents their lives. According to the early 17th century Mikawa Monogatari, Sakai Tadatsugu brought Princess Toku’s twelve complaints to Nobunaga, though does not include what they were, and when Nobunaga asked Tadatsugu about the complaints one by one, he confirmed ten of them. So Nobunaga ordered Tadatsugu to tell Ieyasu to have Nobuyasu commit seppuku. Which he did on October 5, 1579.

Problems with said “evidence”

Japanese scholars in the last few decades have begun questioning every traditional narrative, including this one. Some problems have been identified:

• Was Princess Toku really on such bad terms with Tokugawa Nobuyasu? They were married when they were both nine, grew up together, and had been married for over a decade. Their two daughters were born in 1576 and 1577 respectively, so it certainly doesn’t seem likely they got sick of each other. And considering they were both 21 at the time of his death, it seems a bit premature for anyone to be harping on about sons and heirs.
• Nobuyasu himself must have known the consequence of mistreating Princess Toku, who was in some ways Nobunaga’s representative.
• Killing Nobuyasu seem to be a huge over-reaction even for Nobunaga for a couple’s quarrel.
• Both the accusations leveled at Lady Tsukiyama and Nobuyasu in the Matsudairaki and Mikawa Gofudoki seem to be very generic. He was the blood-thirsty, violent, and barbaric tyrant, though this is in stark contrast with his depiction in the Mikawa Monogatari. She was the hateful wife who cursed her husband, cheated on him with some random doctor, and poisoned the ears of her son, and before her death say she’ll be a ghost and curse all that follows. The early 19th century edited version even have her write to Ieyasu blaming him for her father’s death and instead of pitying her and treating her well afterwards, casting her aside. And that she’s going to be a demon and take her revenge.
• Could a woman, even of her position, actually plot with a foreign power?
• The letter doesn’t even say Nobuyasu was in on the plot, only Lady Tsukiyama was trying to convince him, so why did Nobunaga order Nobuyasu’s death? Why didn’t he order Lady Tsukiyama’s death, or the death of both?
• Nobunaga didn’t order Lady Tsukiyama’s death, so why was she executed?
• According to Mikawa Monogatari, Nobuyasu’s death was basically the fault of Sakai Tadatsugu. The book says so directly and say because of this Tadatsugu lost a lot of respect among the Tokugawa. Why was he then given the overall command in the Shinano campaign in the scramble for territory following Nobunaga’s death.
• And who was to be blamed? Lady Tsukiyama or Nobuyasu or Tadatsugu? While later sources like the early 19th century Mikawaki or Edited Mikawa Gofudoki combines both versions, it’s clear the versions given in these early 17th century sources actually conflict with each other.

One thing that stood out among the scholars was also that these versions of the story conveniently absolves Ieyasu of all blame. It was either that both Lady Tsukiyama and Nobuyasu were bad (Matsudairaki/Mikawa Gofudoki) or Princess Toku for slandering them behind their backs and Sakai Tadatsugu for not just denying the allegations (Mikawa Monogatari), and it was Nobunaga who ordered Nobuyasu’s death. From start to finish, Ieyasu was the victim. This seems a bit too convenient considering they were his wife, heir, daughter-in-law, ranking vassal, and de-facto lord. Surely in reality Ieyasu must shoulder some of the blame.