Siege of Gassan-Toda Castle

The Siege of Gassan-Toda Castle — Mōri Motonari's Unification of the Chūgoku Region and an Impregnable Mountain Fortress

From Eiroku 8 (1565) to Eiroku 9 (1566), Mōri Motonari besieged and conducted a prolonged siege by starvation of Gassan-Toda Castle (located on Toyoda-yama in Izumo Province, now Shimane Prefecture), the stronghold of the Amago clan, ultimately forcing Amago Yoshihisa to surrender. Through this second siege of Gassan-Toda Castle, Mōri Motonari came to exercise effective control over the entire Chūgoku region and established himself as the mightiest daimyō of western Japan.

Gassan-Toda Castle was built on Toyoda-yama in Izumo (184 meters in elevation), a mountain fortress naturally defended on three sides by the Iinashi River. From the late Muromachi period through the Sengoku era, the Amago clan, controlling Izumo, Iwami, and Oki, ruled as a powerful guardian daimyō; at their zenith they commanded approximately eight provinces in the Chūgoku region. As their principal stronghold, Gassan-Toda was renowned as one of the most formidable and impregnable castles in the realm.

Mōri Motonari and the Three Arrows Lesson — The Path to Unification of the Chūgoku Region

Mōri Motonari (1497–1571), though born the son of a minor feudal house in Aki Province (now Hiroshima Prefecture), rose through masterly combination of stratagem, diplomacy, and military prowess to become supreme hegemon of the Chūgoku region. The celebrated account of his instruction to his three sons—his eldest son Takamoto, his second son Motoharu (adopted into the Kikkawa family), and his third son Takakage (adopted into the Kobayakawa family)—known as the "three arrows lesson," wherein he taught them the indissoluble strength of unity, ranks among the most famous paternal injunctions in Japanese history.

Motonari rose to dominance when he defeated Sue Harukata, who held actual power within the Ōuchi clan, through stratagem at the Battle of Itsukushima (Tenbun 24, 1555), thereby seizing the Ōuchi and establishing his hegemony over western Japan centered on Yamaguchi. His next objective was the Amago clan in Izumo.

The First and Second Sieges of Gassan-Toda — Motonari's Trial and Strategy

In fact, the siege of Gassan-Toda Castle by Mōri Motonari was conducted on two separate occasions. The first siege (circa Daiei 3, 1523) occurred in Motonari's youth, when he participated as an auxiliary force for the Ōuchi, but it ended in failure. The second siege (Eiroku 8–9, 1565–1566) was a prolonged blockade undertaken by Motonari himself as the final phase of his conquest of the Chūgoku.

In the second assault, Motonari rejected direct assault and adopted prolonged siege by starvation as his principal strategy. Gassan-Toda Castle, by virtue of its nature as a mountain fortress, presented terrain markedly unfavorable to the attacker, and frontal assault risked catastrophic casualties. Motonari instead chose to seize strategic points surrounding the castle and sever its supply routes, thereby reducing the garrison to capitulation through starvation.

Kikkawa Motoharu (the second son who inherited the Kikkawa house) and Kobayakawa Takakage (the third son who inherited the Kobayakawa house) served as the pillars of the Mōri forces to the east and west, executing the encirclement. This encirclement formed by the "three arrows"—Mōri, Kikkawa, and Kobayakawa—operated continuously for approximately one year, until finally, in Eiroku 9 (1566), with the castle's provisions exhausted, Amago Yoshihisa opened his gates and surrendered in November.

The Amago Clan's Sword Culture — Izumo's Smiths and the Fate of Named Blades

As a daimyō commanding Izumo and Iwami, the Amago clan patronized and cultivated the local sword smiths. Izumo Province had long been known as a source of high-quality iron sand (tamahagane, the fundamental material for Japanese swords), and tatara iron-smelting flourished in the mountainous regions straddling the border of present-day Shimane and Tottori Prefectures. The tamahagane produced by this tatara process represented the finest material available for the craft, and the smiths of Izumo and Iwami forged celebrated blades by drawing upon this superior steel.

With the fall of the Amago and the passage of Gassan-Toda Castle into Mōri hands, the patron of Izumo's smiths changed. The Mōri clan brought the sword smiths of the entire Chūgoku region under their dominion and secured command of the supply system for tamahagane and iron sand across their vast territories. In particular, the Bizen school centered on Osafune (now Okayama Prefecture) came under Mōri influence as their domains expanded.

It is believed that many of the named swords held by the Amago clan were confiscated by the Mōri following Gassan-Toda's fall. The Ōkanehira is a masterwork of Kanehira, a celebrated smith of the Osafune school of Bizen, and tradition holds that it was once possessed by the Amago clan; today it resides in the Okayama Prefectural Museum as a National Treasure. Though its true attribution remains uncertain, it is cited as an emblematic instance of the transfer of swords from the Amago to the Mōri house.

Mōri Motonari's Philosophy of the Sword — The Three Arrows and the Binding of Iron

Mōri Motonari earned his reputation not as a master of the blade but as a commander of strategy, distinguished by his skill in intrigue, diplomacy, and intelligence. Yet Motonari did not disregard swords; historical records attest that he employed the named blades acquired in his conquest of the Chūgoku region as instruments of political authority.

Motonari used gifts of named swords to powerful military leaders and allies as a means of diplomacy, and he bestowed blades upon his retainers as rewards and symbols of honor. The philosophy embodied in the "three arrows lesson"—that a single arrow breaks easily but three arrows bound together cannot—was precisely realized in the reality of Mōri, Kikkawa, and Kobayakawa acting in concert to reduce Gassan-Toda Castle.

The Historical Significance of the Siege of Gassan-Toda — The Establishment of Hegemony in Western Japan and the Transformation of Sword Culture

The historical significance of the Siege of Gassan-Toda lies in the completion of Mōri unification of the Chūgoku region. Through this victory, Mōri Motonari transformed from a minor feudal lord of Aki into the foremost daimyō of western Japan, commanding eight to ten provinces across the Chūgoku.

Furthermore, by bringing under their control Izumo and Iwami—regions of premier tatara iron-smelting—the Mōri clan secured dominion over a critical source of tamahagane, the raw material indispensable for Japanese swords. This resource advantage proved foundational to the Mōri military supremacy in their subsequent struggles against Oda Nobunaga and Toyotomi Hideyoshi (exemplified by the Siege of Takamatsu Castle in Bitchu).

The extinction of the Amago and establishment of Mōri rule over Izumo brought direct transformation to the activities of Izumo and Iwami smiths. The regional sword production that had flourished under traditional Amago patronage was now incorporated into the Mōri's expansive administrative framework, becoming part of a larger integrated network. The traditions of tatara iron-smelting and sword-smithing in Izumo and Iwami persisted through the Edo period and continue to be transmitted to the Japanese sword craftsmen of today.