In this Japanese name, the family name is Tokugawa (徳川).
Born as Matsudaira Takechiyo (松平 竹千代), Ieyasu was the daimyo of Mikawa. At this time, Ieyasu was a vassal for the Daimyo of Totomi, Imagawa Yoshimoto. In 1560, Ieyasu and his retainers, Hattori Hanzō and Honda Tadakatsu defected from Imagawa at the battle of Okehazama to Oda Nobunaga, daimyo of Owari. Ieyasu and Nobunaga spent some of their childhood together, whilst Ieyasu was held hostage. This new alliance would prove a success at the Battle of Nagashino in 1574 against the army of the now dead Takeda Shingen. After the betrayal of Nobunaga's retainer, Akechi Mitsuhide at the Incident at Honnō-ji in 1582, Ieyasu decided to support Nobunaga's eldest son, Oda Nobukatsu allowing the Tokugawa armies to control Owari. But Toyotomi Hideyoshi, now daimyo of Osaka sent his army to invade Owari. This was the only time that there was war between the Toyotomi and Tokugawa clans. The results were that Hideyoshi offered a truce with Nobukatsu, although Hideyoshi did not fully trust Ieyasu. It would be five years before the two would fight again as allies.
In 1590, the Tokugawa-Toyotomi army declared war on Hōjō Ujimasa, daimyo of the Kanto region who refused to accept Hideyoshi's rule. Although he was occasionally Ujimasa's ally, Ieyasu sided with Hideyoshi and was one of the commanders at the siege of Odawara castle. After the Hojo surrendered, Hideyoshi offered Ieyasu the Hojo lands. Ieyasu now made the riskiest choice of his life, giving up his homeland of Mikawa in exchange for his new capital in Edo (Modern day Tokyo). Earlier in his life Ieyasu recorded that Oda Nobunaga planned to invade China. Hideyoshi carried that legacy on in 1592 when he invaded Korea. Ieyasu took no part in the invasion whatsoever. However Hideyoshi was not only failing with the Conquest of Korea, he was dying. Hideyoshi summoned five daimyo to Osaka. These five would rule Japan until Hideyoshi's son, Toyotomi Hideyori was of age. These five were Maeda Toshiie, Mōri Terumoto, Ukita Hideie, Uesugi Kagekatsu and Tokugawa Ieyasu himself. After Hideyoshi's death, Ieyasu immediately made alliances with daimyo who hated Hideyoshi.
With Maeda Toshiie's death in 1599 Ieyasu took Osaka Castle for his own. This saw the emergence of a new influence: a man named Ishida Mitsunari. Mitsunari was a loyal subordinate of Hideyoshi and he hated Ieyasu with immeasurable fury. So much so he even sent ninja to murder Ieyasu which failed. Ieyasu's third son, Tokugawa Hidetada was furious and went with his father's supporters to kill Mitsunari. Oddly enough, Mitsunari was protected by Ieyasu. Being the master strategist he was, Ieyasu preferred that a lesser daimyo lead the enemy army rather than one of the regents. Mitsunari was infuriated causing all of Japan to be split into East vs West. Ieyasu was the de facto leader of the East as were his allies. Ieyasu's new allies included Maeda Toshinaga, Mogami Yoshiaki, Satake Yoshinobu and the "One eyed Dragon of Oshu": Date Masamune.
In 1600, the two factions met for the final battle. Sekigahara was the largest gathering of samurai in Japanese history. Amongst the leaders of the Western army was Hideyoshi's nephew, Kobayakawa Hideaki who had good relations with Ieyasu but was his enemy. When he was ordered to attack the Eastern Army he couldn't decide. So Ieyasu ordered his cannons to fire on Hideaki causing him to defect during the battle. The victory went to the Eastern Army and Tokugawa Ieyasu was finally declared Shogun.