The great warlords Oda Nobunaga and Tokugawa Ieyasu had a long and complicated alliance with each other until Nobunaga's assassination in 1582. Although Nobunaga was the senior and more powerful warlord during that time, their relationship was unique in that Ieyasu did not appear to act as a subordinate vassal to Nobunaga.
Nobunaga made peace with Ieyasu who was the new power in Mikawa province. It was a far-sighted alliance that secured Nobunaga's eastern flank which allowed him to concentrate on his conquests Mino, Nagashima, and Ise provinces and eventually his march on Kyoto. The alliance between Nobunaga and Ieyasu helped them in their efforts against other powerful warlords during that time such as Takeda Shingen and Uesugi Kenshin.
Although they both benefited from their relationship of cooperation, there was a dark side to the coalition between Nobunaga and Ieyasu. The most famous and shocking is that of the forced suicide of Ieyasu's first and formal wife, Lady Tsukiyama, together with their oldest son, Matsudaira Nobuyasu. Nobuyasu had married Nobunaga's eldest daughter Tokuhime in 1567. Initially, the marriage appears to have been a happy one, and the couple had two daughters.
However, tension between Nobuyasu and Tokuhime slowly mounted as years went by, apparently due to the intrigues of Nobuyasu's mother, Tsukiyama. The couple eventually ended up living in complete disharmony. In 1579, Tokuhime sent her father a letter accusing her husband and his mother of conspiring with the Takeda against the interests of the house of Oda. Nobunaga took the accusation very seriously, and drew a drastic but logical conclusion:
Ieyasu had to restore order to the house--which meant that Nobuyasu and Lady Tsukiyama had to die.
Ieyasu now faced an agonizing dilemma: either he sacrificed his wife and son, or, if he was not prepared to do so, put his partnership with Nobunaga on the line. The first option would be a great personal tragedy, but to take the latter course would jeopardize the further existence of the entire house of Tokugawa, and might undo everything he had achieved over the past two decades.
At the end of what must have been long and painful deliberations, Ieyasu condemned his eldest son and his first wife to death. Ieyasu, it seems, was prepared to pay a high price for his alliance with Nobunaga.
Japonius Tyrannus by Jeroen Lamers