The Two-River Structure: How Motonari Came to Rule Ten Provinces

Mōri Motonari's conquest of the Chūgoku region was not achieved by military victory alone.

The core, rather, lay in a 'strategy of blood' through marriage policy and adoption.

Sending his second son Kikkawa Motoharu in adoption to the Kikkawa house, and his third son Kobayakawa Takakage to the Kobayakawa house, he constructed the 'Two-River structure' (ryōsen taisei) that absorbed both houses — and that is what made possible the Mōri rule of the ten provinces of the Chūgoku region.

It is the true distinguishing mark of Motonari as a Sengoku daimyō.

When Motonari succeeded in 1523, the Mōri house was no more than one of several middling kokujin lords centered on Yoshida District in Aki Province.

The force they could mobilize was a few thousand, the holdings were within part of Aki Province, and they stood in a severe position between the surrounding great powers (the Ōuchi of Suō and the Amago of Izumo).

The expansion to a great domain ruling ten provinces by Motonari's late years, thirty years later, was an astonishing rise — but the starting point was by no means advantageous.

The core of Motonari's strategy was not military conquest but the absorption of powerful kokujin lord houses around him through adoption.

Sending his second son Motoharu (then twelve) in adoption to the Kikkawa house, a great Aki house, in 1547 (Tenbun 16); and sending his third son Takakage (then twelve) to the Kobayakawa house, a powerful Aki kokujin, in 1550 (Tenbun 19).

The former heads of both houses retired from the headship through complex courses, and Motoharu and Takakage, bearing Mōri blood, succeeded to both houses.

By this the Kikkawa and Kobayakawa houses became, in substance, branches of the Mōri, and the firm structure of three houses in confederation was completed.

The Two-River structure was not merely a blood relationship but an operational system with clear division of roles.

The main house Mōri took the center (with Yoshida-Kōriyama Castle as the base); Kikkawa Motoharu took charge of the San'in direction (the campaigns against Amago in Izumo, Hōki, Inaba, and so on); Kobayakawa Takakage took charge of the San'yō direction (Bitchū, Bingo, Iyo, and the Inland Sea direction).

While the three houses could each take independent military action, the overall strategy was governed by Motonari — an extremely advanced division-of-labor structure as a Sengoku-period retainer-body organization.

In parallel with the Two-River structure, Motonari incorporated the kokujin lords within Aki Province into the Mōri retainer body in stages.

The purge of arbitrary powerful retainers including Inoue Motokane (1550), the collection of blood-sealed pledges guaranteeing unity, the securing of loyalty by granting of holdings — he systematically maintained the methods of retainer-body control.

By this the Mōri ruling structure, which had been a confederation of kokujin lords, was transformed into a Sengoku daimyō house of main-house central authority.

Motonari, who toppled Sue Harukata at the Battle of Itsukushima (1555), absorbed the former Ōuchi territory (Suō and Nagato) at once.

He then established control of the Iwami silver mine (1561), and reduced the Amago house at Gassan-Toda Castle to take Izumo (1566).

At the time of Motonari's death in 1571, he was a great daimyō, among the foremost of the Sengoku, ruling a great domain of ten provinces — Aki, Suō, Nagato, Iwami, Bingo, Bitchū, Izumo, Oki, Hōki, and Inaba.

The force he could mobilize reached the tens of thousands.

After Motonari's death, his grandson Terumoto succeeded as head, and the Two-River structure was maintained.

In Toyotomi Hideyoshi's unification of the realm, the Mōri house was confirmed as a great daimyō of the Chūgoku region, and Terumoto became one of the Five Elders.

At the Battle of Sekigahara in 1600 Terumoto was set up as commander of the Western Army but did not move from Ōsaka Castle, and after the battle was reduced to the two provinces of Suō and Nagato (Hagi at 360,000 koku).

Even so the house survived and continued as the Chōshū Mōri house until the Meiji Restoration.

The Mōri house (the Chōshū domain), greatly reduced at Sekigahara, harbored resentment against the Tokugawa bakufu throughout the Edo period, and became the core of the movement to topple the bakufu in the Bakumatsu.

Yoshida Shōin, Takasugi Shinsaku, Itō Hirobumi, Kido Takayoshi (this site's persons 36-38, 47) are all from the Chōshū domain.

The structure and spirit of the Mōri house that Motonari built became the force that drove the Meiji Restoration three hundred years later.

It is a rare case in which the legacy of a single Sengoku daimyō reached into the birth of the modern Japanese state.