Tokugawa Nobuyasu was the eldest son of Tokugawa Ieyasu, the son of Tokugawa Ieyasu's wife, Chikuyama Densei. We know that the eldest son was very important in ancient China, and he will be the natural first heir, or even the only, heir of the princes and generals. Japan is deeply influenced by Chinese culture and attaches great importance to the eldest son. However, Tokugawa Ieyasu ordered his eldest son, Tokugawa Nobuyasu, to commit suicide.
Of course, Tokugawa Ieyasu forced his eldest son to commit suicide, and he himself was the result of persecution. In other words, someone forced Tokugawa Ieyasu to kill his eldest son, and Tokugawa Ieyasu had no choice but to do so.
(Tokugawa Ieyasu)
So, who had such a great ability to force Tokugawa Ieyasu to kill his eldest son, and he had no power to fight back?
To make this clear, we have to start at the beginning.
Tokugawa Ieyasu was originally surnamed Matsudaira and was called Matsudaira Motoyasu. The Matsudaira family was also the daimyō of the Mikawa Kingdom during the Sengoku period of Japan, and although it was already quite weak by the time it reached Tokugawa Ieyasu's father, Matsudaira Hirutaka, relying on the powerful daimyō Imakawa Yoshimoto at the time, was able to gain a foothold in the Mikawa Kingdom. The price was to send Tokugawa Ieyasu to Imakawa Yoshimoto as a hostage.
Tokugawa Ieyasu married Imakawa Yoshimoto's adopted daughter Chikusanden and gave birth to his eldest son, Tokugawa Nobuyasu (then known as Matsudaira Nobuyasu).
Yoshimoto Imagawa had a major battle with another powerful daimyo, Oda Nobunaga, in the course of his development. During this great battle, Tokugawa Ieyasu secretly joined forces with Oda Nobunaga and almost completely annihilated Imakawa Yoshimoto's forces, and Imagawa Yoshimoto was also killed. Tokugawa Ieyasu was able to return home with his wife and children and return to the Three Rivers Kingdom to inherit the position of governor of the family.
The most important reason why Tokugawa Ieyasu was able to become independent and became large was because he was attached to Oda Nobunaga. Tokugawa Ieyasu prepared to marry Oda Nobunaga in order to continue to grow, and at his request, Oda Nobunaga married his daughter Tokugawa Nobunaga to Tokugawa Nobunaga.
According to the strength of Oda Nobunaga at that time, in a sense, Tokugawa Nobunaga was equivalent to a horse master. Generally speaking, because the princess is quite strong, it is not easy to do it. What's worse is that Tokugawa Shinyasu returned the concubine, and the relationship with the concubine was much better than the relationship with Tokuhime. At the same time, as a mother-in-law, the mountain temple is not satisfied with Tokuhime.
(Tokuki)
As a result, Tokuhime was furious and ran back to her mother's house to cry to her father, Oda Nobunaga.
In fact, if Tokuhime had only cried to Oda Nobunaga that her husband and mother-in-law were not good for her, it is estimated that Oda Nobunaga would be angry, but not so angry. The point is that in order to impress her father, Tokuhime fabricated that Chikuyama-den was secretly in contact with the Takeda clan of Kai Kingdom and wanted to unite with the Takeda clan and attack Oda Nobunaga.
You know, although Oda Nobunaga's power at that time was as great as the sun, it was not without opponents. The Takeda clan of Kai Washi was dissatisfied with the Oda clan and constantly challenged Oda Nobunaga. This made Oda Nobunaga very angry. Now that her daughter has told herself that the Tokugawa and Takeda families are secretly united, how can Oda Nobunaga not be angry?
Oda Nobunaga was justified in his suspicions of Tokugawa Ieyasu. Originally, the Tokugawa family was united with Imagawa Yoshimoto, and Tokugawa Ieyasu dared to unite with Oda Nobunaga and oppose Mizumi Imakawa Yoshimoto. Now why can't you unite with the Takeda family and oppose the water yourself!
Oda Nobunaga then sent a letter asking Tokugawa Ieyasu to kill Chikuzan-den and Tokugawa Nobunaga.
Tokugawa Ieyasu was taken aback when he received the letter. Although Tsukuyama Hall was his original match, he and Tsukuyama Hall had long since lost their feelings, and even if he killed him, he would not be very distressed. But Tokugawa Nobuyasu was his eldest son, so how could he be willing to kill him? So he wrote back to Oda Nobunaga, saying that the incident may have been done by Chikuyama-den alone, and Thatogawa Nobuyasu was still young, and he certainly did not know it.
(Tokugawa Nobuyasu)
Tokugawa Nobuyasu wanted to "give up the car to protect the marshal", but Oda Nobunaga was an understanding person, and he directly blocked the road, saying, even if Tokugawa Nobuyasu did not know, but I killed his mother, and he became the governor of the house, will he spare me? In short, whether he knew it or not, he had to die.
The Tokugawa family had no choice but to kill Chikusan-den and force Tokugawa Nobuyasu to commit suicide.
Tokugawa Ieyasu killed his eldest son, although he was coerced. But, after all, it was his own son. Oda Nobunaga's alliance was important, but for the sake of his son, couldn't Tokugawa Ieyasu go against Oda? Besides, he is not alone, he can really unite with the Takeda family, he may not be defeated.
It was precisely because the Tokugawa family's killing of their son was so bizarre that there was a lot of speculation about it in later generations. One of the most influential theories is said in the Tale of the Three Rivers. Because Chikuzanden was not favored by Tokugawa Ieyasu, she secretly contacted the Takeda clan and wanted to unite with the Takeda clan, kill Tokugawa Ieyasu, and help her son Tokugawa Nobuyasu to the throne. This was discovered by Tokugawa Ieyasu, who killed his wife and son with the help of Oda Nobunaga.
However, this kind of "power struggle theory" has been proved incorrect by later scholars after examination.
In fact, from the Fact that the Tokugawa family eventually became the final victor in the Sengoku period, it can also be seen that in order to succeed, he did everything and endured any grievances. It is also very likely that he forced his eldest son to die.
(References: The Tale of Three Rivers, Matsudaira)