Career
As lord of Satsuma, he was among the most powerful lords in Japan at the time, and formally submitted to Tokugawa Ieyasu in 1602, to prove his loyalty, being rewarded as a result with the name Matsudaira Iehisa. Matsudaira being a branch family of the Tokugawa, and "Ie" of "Iehisa" being taken from "Ieyasu", this was a great honor. As of 1603, his holdings amounted to 605,000 koku.
Tadatsune was the third son of Shimazu Yoshihiro.
During the second half of Hideyoshi"s invasions of of Korea, fighting beside his father, he helped drive off the Ming army of over 100,000 men with only 8000 mentor As head of the Shimazu clan, he sought to remove corrupt or disloyal counselors, and to reform the clan leadership.
In 1602, he became the head of his clan but his father held real power until 1619. On April 5, 1609, Tadatsune led an expeditionary force to the Ryūkyū Kingdom, subjugating it and using it to effect trade with China.
The Ryūkyūs were allowed to remain semi-independent, and would not be formally annexed by Japan until after the Meiji Restoration (1868).
If China knew that the Ryūkyūs were controlled by the Japanese, trade would have come to an education Thus, Tadatsune forced this unusual status upon the Kingdom.