Lifespan: Chōkyō 2 (1488) to 6/19 of Eiroku 8 (1565)
Rank: sengoku daimyō
Title: Junior Fourth Rank (Lower), Master of the Eastern Capital Office
Bakufu: Muromachi – Military Governor of Mutsu
Father: Date Hisamune
Siblings: Tanemune, Rusu Kagemune, Hisamatsumaru, sister (second wife of Mogami Yoshisada)
Wife: Taishin-in (daughter of Ashina Moritaka), daughter of Nakajō Sadasuke, daughter of Watari Munetaka
Children: Yakata Gozen, daughter (formal wife of Ashina Moriuji), Harumune, Ōsaki Yoshinobu, Sanemoto, daughter (wife of Nikaidō Teruyuki), daughter (wife of Tamura Takaaki), Munezumi, daughter (wife of Kakeda Toshimune), Koori Munesada, Kasai Harukiyo, Yanagawa Munekiyo, Murata Munetane, Gokurakuin Sōei, Watari Tsunamune, Watari Motomune, Ōari Yasuura, Koshikawa Gozen (formal wife of Sōma Yoshitane)
Date Tanemune served as a sengoku daimyō in Mutsu Province. He held the title of Junior Fourth Rank (Lower) and Master of the Eastern Capital Office. Tanemune served as the fourteenth head of the Date family.
In 1488, Tanemune was born as the eldest son of Date Hisamune, the thirteenth head of the Date family. Based on custom, he received on the characters from the name of Ashikaga Yoshitaka (the eleventh shōgun, later known as Ashikaga Yoshizumi) and initially adopted the name of Takamune.
In 1514, after the death of Hisamune, Tanemune succeeded him as the fourteenth head of the Date family. Later that year, he defeated Mogami Yoshisada, the ushū–tandai, or Commissioner of Dewa, for the Muromachi bakufu, at Hasedō Castle. Tanemune then sent his younger sister to become the wife of Yoshisada in a political marriage and, in effect, placed the Mogami clan under his command. In 1517, for purposes of celebrating the arrival in the capital of Ashikaga Yoshitane (the tenth shōgun who returned to his position after the dethronement of Ashikaga Yoshizumi), Tanemune contributed gifts of significant value. Through Hosokawa Takakuni (the kanrei, or deputy shōgun), he requested and was granted permission to use one of the characters from the name of Yoshitane in his own name, upon which he adopted the name of Tanemune. In addition, he was appointed as the Master of the Eastern Capital Office. Originally, this title had been inherited over successive generations by members of the Ōsaki clan who served as the commissioners of Oushū (meaning Mutsu Province), but, by virtue of the Date clan acquiring this title, it demonstrated that the Date had taken the place of the Ōsaki in name and in fact. In this manner, Tanemune leveraged his connections with the central authorities of the Muromachi bakufu to raise the status of the Date family. At the same time, he fought against the Kasai and Iwaki clans, and wove-in marriage diplomacy to rapidly expand his regional power.
In 1520, after Mogami Yoshisada died without an heir, assorted commanders in the Mogami clan rebelled after growing weary of the exercise of influence by Tanemune through Yoshisada’s second wife (a daughter of Date Hisamune). This led to a confrontation between the Date and Mogami clans. Acting with great intensity, Tanemune toppled Kaminoyama, Yamagata, Tendō, and Takadama castles, and then attacked Sagae. At this time, the Date consolidated forces from the Kasai, the Sōma, the Iwaki, the Aizu, the Miyagi, the Kokubun, and the Mogami. The encampment stretched from Takaseyama to Hachimanbara. While the forces remained in place over a one-month period, a settlement was reached between the Date and Sagae clans so the Date clan withdrew without any fighting. As a result of this action, the Mogami District and the southern portion of the Murayama District came under the control of the Date clan.
In 1522, the Muromachi bakufu took the unprecedented action of appointing Tanemune as the military governor for Mutsu Province. In the twelfth month of the same year, a return gift arrived from Ashikaga Yoshiharu (the shōgun) to celebrate the beginning of a new era. In 1523, Tanemune was ordered to donate to the construction of the Iwa-Shimizu Hachiman Shrine in Kyōto.
In 1532, after moving his base from Yanagawa Castle to Koori-Nishiyama Castle, he worked to strengthen his governance. In 1533, he began by enacting thirteen articles in addition to preparing tax collection registers in 1533 and 1538. In 1536, he enacted a provincial code containing 171 provisions with the aim of expanding the governing organs of the Date clan. That same year, to suppress an internal rebellion in the Ōsaki clan, Tanemune accepted a request from Ōsaki Yoshinao to provide reinforcements along with lords from southern Oushū (Mutsu). In return, Tanemune had Yoshinao accept his second son, Yoshinobu, as his heir. As a result, Tanemune succeeded in having the role of commissioners for the bakufu for both Mutsu and Dewa provinces under the command of the Date clan.
Despite the progress achieved, Tanemune faced deepening opposition from within the family. His eldest son, Date Harumune, along with senior retainers including Koori Kagenaga and Nakano Munetoki, disfavored a plan by Tanemune to have his third son, Date Sanemoto, become the heir to Uesugi Sadazane (the military governor of Echigo) in addition to a plan to allocate a portion of the Date territory to his son-in-law, Sōma Akitane. In the sixth month of 1542, while returning from a falconry outing, Tanemune was unexpectedly attacked by Harumune, apprehended, and incarcerated at Nishiyama Castle. Before long, he was rescued by a retainer named Koyanagawa Munetomo. Tanemune called together the lords of southern Oushū and made preparations for battle against Harumune, giving rise to the Tenbun Conflict that engulfed all of Oushū. Initially, Tanemune enjoyed the advantage, but, in 1547, after Ashina Moriuji switched sides to support Harumune, the circumstances turned against him. In the ninth month of 1548, through the mediation of Ashikaga Yoshiteru (the thirteenth shōgun), Tanemune surrendered to Harumune, whereby Tanemune was compelled to assign the headship of the clan to Harumune and to retire to Marumori Castle. As a consequence of this conflict extending for six years, local clans including the Ōsaki, the Kasai, the Mogami, the Sōma, and the Ashina all intervened, raising their level of influence among the Date, and transcending their prior roles as subordinates to the family. There is also a theory that the conflict with Harumune persisted into the Kōji era (1555 to 1558).
On 6/19 of Eiroku 8 (1565), Tanemune died at the age of seventy-eight at Marumori Castle. He was interred at the Yōrin Temple were Tanemune himself had laid the foundation for its construction. His loyal retainer, Koyanagawa Munetomo, martyred himself in front of Tanemune’s memorial.