He was born a son of Sakai Tadachika, of a house that had served the Matsudaira of Mikawa for generations. The Sakai were senior retainers long tied to the Matsudaira. About fifteen years older than Ieyasu, Tadatsugu would support the Matsudaira from the time of Ieyasu's father.
When the young Takechiyo, later Ieyasu, was sent to Sunpu as an Imagawa hostage, Tadatsugu is said to have accompanied him. Dates vary. In hard years without a lord at home, the retainers endured under the Imagawa, waiting for the Matsudaira revival.
When Imagawa Yoshimoto was killed at Okehazama, Ieyasu returned to Okazaki Castle and broke from the Imagawa. As a senior retainer Tadatsugu supported him, working for the alliance with Oda Nobunaga and the pacification of Mikawa. Their long journey as lord and vassal began.
Ikko followers in Mikawa staged a great uprising, and many of Ieyasu's retainers joined it. Even some of the Sakai clan sided with the rebels, but Tadatsugu stayed with Ieyasu. Overcoming this war, Ieyasu rebuilt his body of retainers.
He was entrusted with Yoshida Castle, today's Toyohashi, key to eastern Mikawa, and became leader of its local lords. Ishikawa Ienari, later Kazumasa, led western Mikawa, and Ieyasu's army was organized around the two as its twin wheels.
With Oda Nobunaga he fought the Azai-Asakura allied army at the Anegawa in Omi, battling hard in the Tokugawa vanguard. By sending troops far for their ally Nobunaga, the Tokugawa deepened the Oda's trust.
The Tokugawa army was badly beaten by Takeda Shingen's great host at Mikatagahara. A tale says that after Ieyasu fled back to Hamamatsu Castle, Tadatsugu beat the castle drum to rally his side and make the pursuing Takeda wary, though it is not a reliable record.
At Nagashino he proposed a night flanking raid on the Takeda fort at Tobinosuyama, led the detached force himself and took it. Cut off from retreat, the Takeda came out to Shitaragahara for a decisive battle, leading to a great Oda-Tokugawa victory.
Ieyasu's eldest son Nobuyasu, suspected by Nobunaga, was driven to take his own life. One account holds that Tadatsugu went as envoy to explain to Nobunaga but could not clear the suspicion, but the course of events is disputed and remains unclear.
In the Komaki-Nagakute campaign between Ieyasu and Hashiba Hideyoshi, he attacked and routed Mori Nagayoshi's force at Haguro in Owari. This victory helped tilt the course of the campaign toward the Tokugawa and showed his skill as a veteran general.
Partly due to eye disease, he handed headship to his son Ietsugu and came to live in Kyoto. Toyotomi Hideyoshi is said to have granted him a residence and land for his living, showing he was respected by the Toyotomi government as Ieyasu's senior retainer.
He died in Kyoto. Having supported Ieyasu in war and diplomacy from his youth, Tadatsugu was later counted with Honda Tadakatsu, Sakakibara Yasumasa and Ii Naomasa as the Four Heavenly Kings of the Tokugawa, and is ranked first among them. His descendants ruled the Shonai domain.
Imagawa YoshimotoTakeda ShingenOda NobunagaToyotomi HideyoshiHonda TadakatsuSakakibara YasumasaIi Naomasa
Shibata KatsuieMiyoshi NagayoshiTakigawa KazumasuHachisuka MasakatsuAkechi MitsuhideRyuzoji TakanobuUesugi KenshinŌtomo Sōrin
People of the Sengoku/Azuchi-Momoyama period ・ People by period
Based on facts found in textbooks and biographies. Uncertain points are marked as such.