Date: 9/20 of Keichō 5 (1600) to 4/26 of Keichō 6 (1601)
Location: Iwasaki Castle in the Waga District of Mutsu Province
Outcome: With the support of the Date army, Waga Tadachika and his supporters made initial progress toward his aim to recapture his former territory. Eventually, however, he was cornered in Iwasaki Castle and unable to withstand a vigorous assault by the infantry of the Nanbu army after the attacking forces set fire to the castle.
The Iwasaki Uprising occurred in 1600 when Waga Tadachika launched a rebellion against the Nanbu clan in a bid to recover his former territory. The actions were instigated by Date Masamune who sought to expand his own territory. This event is also referred to, among other names, as the Battle of Iwasaki, the Iwasaki Campaign, the Waga Uprising, the Waga War, and the Waga-Iwasaki Uprising.
An earlier uprising by the Waga and Hienuki clans in late 1590 (known as the Waga-Hienuki Uprising) based on grievances with the outcome of the Oushū Retribution implemented by the Toyotomi administration in the Tōhoku Region was suppressed by the Toyotomi army while the disputed territory was seized and given to the Nanbu clan. Having lost his fief, Waga Tadachika turned to Date Masamune for protection and was granted land in Hirasawa in the Isawa District of Mutsu Province. Thereafter, Masamune noted that the province was divided in half, and that if Tadachika soon excised territory, his rights would be recognized under the new circumstances, so he should rebel. Moreover, in the event of a battle against the Nanbu clan, he would have Shiroishi Munenao, the lord of Mizusawa Castle, support him. Based from Futago Castle, Tadachika gathered former retainers and remnants from the Hienuki clan to launch surprise attacks against castles aligned with the Nanbu clan in a bid to recover his former territory.
Invasion by ikki forces and counterattack by the Nanbu army
When Tadachika rebelled, the main force of the Nanbu clan was deployed to the Battle of Keichō Dewa. This meant there were fewer military resources available than usual. On 9/20 of 1600, Tadachika took advantage of this opening to launch surprise attacks against Hanamaki Castle (known as the Nighttime Attack on Hanamaki Castle) as well as secondary castles in the area. The attacking forces gained control of the third and second citadels, pressuring the main citadel, but were finally repelled by the valiant fighting of Kita Nobuchika and Kashiyama Akisuke, along with Kita Nobukage who rushed in support. Meanwhile, a unit comprised of former retainers of the Hienuki, backed by the Date army, attacked Ōhasama Castle, toppled Tanaka Tōshirō (a commander of the Nanbu clan), and temporarily seized control of the castle, but, owing to a deterioration of the battle situation, the forces abandoned the castle and fled.
The Nanbu army including Kita Nobukage engaged in a spirited pursuit. After losing numerous fortresses as well as their base at Futago Castle, the ikki forces were ultimately holed-up in Iwasaki Castle from which attempts were made to intercept the Nanbu army. In the middle of the tenth month, the main force led by Nanbu Toshinao who had been readying armaments in Sannohe arrived at Hanamaki Castle. However, owing to the arrival of winter, the season for heavy snow accumulation was upon them, so the fighting was suspended and the army waited until spring to recommence their operations. Meanwhile, Mogami Yoshiaki reported to Tokugawa Ieyasu the details of the uprising.
Fall of Iwasaki Castle and end of the uprising
On 1/20 of Keichō 6 (1601), the Nanbu army held a military council to draw-up their battle plans. On 3/17, the army established a base at the Nanaori fortress near Iwasaki and commenced attacks. The main force including Sakuraba Naotsuna and others attempted to penetrate the main citadel, but were blocked by rectangular-shaped earthen mounds along with counterattacks by the defenders. On 4/4, a unit led by Suzuki Shōgen Yoshinobu (a retainer of Shiroishi Munenao) joined and fought against the Nanbu army. The Nanbu forces were pressed into a bitter fight but, on 4/26, based on a proposal by Kita Nobuchika, the attackers set the castle on fire and followed-up with an all-out assault by the infantry, resulting in the fall of Iwasaki Castle. Waga Tadachika attempted to flee, but, later, along with close retainers including Kamata Harumichi, Tsutsui Kisuke, and Saitō Jūzō, killed himself and was interred at the Kokubun-ni Temple in Mutsu. Under another theory, he was later killed by Date Masamune.
As recognition for his contributions on the battlefield, Kashiyama Akisuke was assigned to Iwasaki Castle; and, owing to his support for the uprising, Shiroishi Munenao was sent to Tome. Meanwhile, Tokugawa Ieyasu viewed the involvement of Date Masamune in the event as troubling so he scrapped a guarantee earlier made in a letter that gave permission to Masamune to recover through his own devices the former territory of six districts totaling 490,000 koku that had earlier been seized by Hideyoshi and conveyed to the Uesugi at the time that Masamune was demoted and transferred to Iwatesawa Castle (later known as Iwadeyama Castle). This is known as the Guarantee (from a superior) of 1,000,000 koku.