Sakai Tadatsugu (酒井 忠次, 1527 – 17 December 1596) was one of the most favored and most successful military commanders serving Tokugawa Ieyasu in the late Sengoku period.

[7] Sakai Tadatsugu was also allegedly involved in the conspiracy that caused the death of Lady Tsukiyama and her son, Matsudaira Nobuyasu.

[3] At some point, Tadatsugu was commanded by Imagawa Yoshimoto to attack Tada Yasumitsu, an uncle-in-law of Matsudaira Hirotada.

[citation needed] In 1556, according to the records from Tosho Gunkan, Tadatsugu defends Fukutani castle which was besieged by 2,000 cavalry troops of Oda clan led by Shibata Katsuie.

[citation needed] In 1560 at the Siege of Marune, Tadatsugu served as vanguard of Tokugawa forces along with Ishikawa Ienari.

[18] Sometime before 1564, the same year, Tadatsugu married princess Usui (Usui-Hime), grand-daughter of Matsudaira Hirotada and sister to Ieyasu.

][21][22][23] Then in the same year, before the Battle of Azukizaka, Tadatsugu wrote letter to the Ikko-Ikki faction in Mikawa, chastising their rebellious conduct.

[26][27][28] Then Ieyasu assigned Tadatsugu, Ishikawa Kazumasa, Sakakibara Yasumasa, and Honda Tadakatsu as chiefs of Tokugawa counsel staffs.

[42][43] In June 1574, when the Takeda clan laid siege to Takatenjin, Oda Nobunaga personally lead reinforcements where he encamped in Yoshida castle and was greeted by Tadatsugu.

The battles were exclusively limited to spear skirmishes outside the wall, which frustrated Katsuyori, causing him to abandon the siege.

[45] Later in the same year, during the campaign in Nagashino, during the war council discussion before the battle, Tadatsugu expressing his idea about night raid, which was quickly rejected by Nobunaga.

[48] Tadatsugu then was tasked to lead a night raid against the Takeda forces located in Tobinosuyama along with Kanamori Nagachika.

Tadatsugu played role in confirming Oda Nobunaga suspicion of the alleged betrayal against the Oda clan being planned by Lady Tsukiyama, thus Nobunaga concluded that if a high-rank Fudai daimyō such as Tadatsugu confirmed the accusation by testifying against Lady Tsukiyama, then her treason must be true.

[8] While Arthur Lindsay Sadler theorized this is deliberate act of spite from Tadatsugu due to his dislikes towards Nobuyasu.

[56] In 1582, after Honnō-ji Incident, Tadatsugu accompanied Ieyasu in arduous journey to escape the enemies of Nobunaga in Sakai and returning to Mikawa.

However, the Hōjō clan in the Kantō region also led an army of 55,000 men and crossed the Usui Pass to invade Shinano Province.

However, Tadatsugu and Nobumine met with unexpected resistance from Suwa Yoritada, a former Takeda vassal who now allied with the Hōjō clan.

Tadatsugu then led a desperate breakthrough and breaking the encirclement and retreat into Wakamiko in Kai province, where he rejoin the Ieyasu's main forces.

Tadatsugu joined Okudaira Nobumasa and Matsudaira Ietada (Fukōzu) in Komaki to launch the attack at dawn.

Tadatsugu, Okanabe Mori, and Yamaguchi Shigemasa spearheading the attack towards Shimojima castle, while Sakakibara Yasumasa, Osuga Yasutaka were deployed to capture any fleeing defenders.

Tadatsugu went into retirement, but his son Ietsugu received a 30,000 koku fudai fief at Usui, in Shimōsa Province, and Tadasugu accompanied them there.

Masazane also authored Tonbōgiri (蜻蛉切 the Dragonfly Slayer), the most famous of the Three Great Spears of Japan (天下三名槍), favourite weapon of Honda Tadakatsu, another one of the Tokugawa Shitennō.

[87] Aside from that, there are another Tachi sword possessed by Tadatsugu which forged by Nagamitsu, pupil of renowned swordsmith named Sanemitsu.