Lifespan: Tenshō 4 (1576) to 5/24 of Keichō 6 (1601)
Title: Assistant of the Bureau of Imperial Cavalry
Father: Waga Yoshitada
Siblings: Yoshinaga, Hidechika, Tadachika, Tadakatsu, Nagakatsu
Wife: Daughter of Ohara Tadahide
Children: Yoshihiro, Ohara Tadahiro
Waga Tadachika served as a bushō during the Azuchi-Momyama period.
The Waga clan were kokujin or provincial landowners, governing the Waga District in Mutsu Province.
In 1576, Tadachika was born as the son of Waga Yoshitada.
In 1590, Toyotomi Hideyoshi raised an army for the Conquest of Odawara. From the second to seventh months, Hideyoshi led the campaign with the aim of subduing the Gohōjō clan based at Odawara Castle in Sagami Province. Landowners and daimyō from the Kantō and Ouu regions deployed one after another to Odawara to join the Toyotomi army, but Yoshitada refused to join, along with others including Yūki Yoshichika, Ishikawa Akimitsu, Esashi Shigetsune, Kasai Harunobu, Ōsaki Yoshitaka, and Hienuki Hirotada (Shigetsuna) who witnessed their territories seized and were expelled from their castles during the subsequent Oushū Retribution.
Former retainers of fallen daimyō from the Ōsaki, the Kasai, and the Kashiyama from the Iwasa District along with local peasants harbored grievances toward the Toyotomi over land surveys conducted by the Toyotomi administration. Soon after the withdrawal of the Toyotomi army, in the tenth month of 1590, these groups launched uprisings across their territories, killed a bushō sent by Hideyoshi named Kimura Yoshikiyo, and wielded their power. In tandem with these events known as the Kasai-Ōsaki Uprising, Yoshitada and Hienuki Hirotada launched their own rebellions in the Waga and Hienuki districts of Mutsu known as the Waga-Hienuki Uprising which was later suppressed by the Toyotomi army.
Later, Tadachika turned to Date Masamune and resided in the Isawa District in the Date territory. In 1600, at the time of the Battle of Sekigahara, Masamune sought to expand his territory so he gave secret orders to Tadachika, whereupon Tadachika launched the Iwasaki Uprising in a bid to recover the Waga District. With the support of Masamune, Tadachika invaded the Waga District governed by Nanbu Yoshinao and stormed Hanamaki Castle (earlier known as Toyagasaki Castle) in addition to outlying castles in an event known as the Nighttime Attack on Hanamaki Castle). Owing to a counterattack by Toshinao and Kita Nobuchika, Tadachika was defeated. Thereafter, he fled safely but then, together with close retainers including Kamata Harumichi, Tsutsui Kinosuke, and Saitō Jūzō, took his own life at the Kokubunni Temple in Mutsu.
Under another theory, Tadachika offered to commit seppuku on behalf the Date family or he may have been assassinated by Masamune. His grave, along with those of seven close retainers, stands at the Kokubunni Temple in the Wakabayashi Ward in the City of Sendai.
As a result of these events, a sealed promise by Tokugawa Ieyasu to grant Masamune an increase of 1,000,000 koku to his fief for his support in the Battle of Sekigahara was scrapped. Tadachika’s eldest son and heir, Waga Yoshihiro, later served the Date clan for a fief of 120 koku.