Mori Motonari (1497–1571), head of the powerful Mori clan, was one of the most influential daimyo in the Chugoku region, located in western Honshu. During the turbulent and violent Sengoku Jidai period, he waged continuous wars in an effort to expand his domain and strengthen his clan's power.
As early as 1543, Motonari had already attempted to seize Gassan-Toda Castle from his neighbor, Amago Haruhisa, but that attempt ended in failure. However, in 1561, Haruhisa died, and two years later, Motonari launched a new military campaign with the aim of conquering Izumo Province.
In order to isolate the enemy's main fortress, Motonari began to systematically capture the remote fortifications belonging to the Amago clan. Among them, the most significant was Shiraka Castle, the largest of the satellite castles (shijo), which was of key importance for defending the approaches to Gassan-Toda. In the fall of 1563, Motonari began his siege. The new head of the Amago clan, Yoshihisa, sent troops to help the besieged, but they were intercepted by Mori's army and defeated. The siege of Shirakawa lasted 70 days and ended in surrender after the besiegers managed to cut off the castle's water supply. The commander of the garrison, Matsuda Michihisa, chose to commit suicide rather than surrender to the enemy.
In the spring of 1564, Motonari approached the walls of Gassan-Toda. This citadel, which belonged to the Amago clan, was one of the five largest mountain castles of its time. Although Gassan-Toda was a yamajiro (mountain castle), it had a highly developed system of defensive structures. In addition to the traditional three courtyards—defensive lines located at the top of Mount Gassan—the fortress had other courtyards that descended in terraces down to the Toda River (now known as Iinashi) and its tributary.
The first attempt to storm the castle, protected by natural conditions and powerful fortifications, was unsuccessful. Mori's army suffered losses, and Motonari was forced to retreat to regroup his forces. In the summer of the same year, he returned to Gassan-Toda with an army of 25,000 samurai. At that time, the castle garrison numbered about 10,000 soldiers. This time, Motonari abandoned the idea of a direct assault and decided to wear down the fortress.
A long blockade began. Motonari issued an order not to accept defectors, thereby contributing to the rapid depletion of food supplies among the besieged. At the same time, he carried out a cunning political maneuver, using the well-known strategy of “killing with someone else's knife.” The head of the Amago clan, Yoshihisa, had a loyal vassal, Uyamahisakane, who was known as a skilled strategist and wise advisor. Motonaori, with the help of his agents, spread rumors among Yoshihisa's people that Hisakane was allegedly preparing to betray him. The false accusations achieved their goal — Yoshihisa believed them and ordered the execution of his valuable assistant. This act seriously undermined the morale of the castle's defenders.
Ultimately, when Motonari lifted the ban on accepting defectors, thousands of exhausted and half-starved Amago warriors abandoned their positions and fled the fortress. By the beginning of winter 1565, the situation inside the castle had become hopeless, and Yoshihisa decided to surrender. To the surprise of many of his contemporaries, Motonari showed mercy and spared the life of his defeated enemy. Yoshihisa was allowed to become a monk and go into exile at Enmyoji Temple in Aki Province.
See also
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Battle of Tennoji
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Siege of Shuri Castle
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The Siege of Fushimi Castle
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The Siege of Otsu Castle
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The Siege of Shiroishi Castle
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The Second Siege of Jinju Castle
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The Siege of Takamatsu Castle
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