Battle of Kizugawa Mouth

What Was the Battle of Kizugawa Mouth?—How Naval Warfare Changed Sengoku History

The Battle of Kizugawa Mouth, conducted twice in Settsu Province's Kizugawa Mouth (present-day Osaka Bay area) in Tenshō 4 (1576) and Tenshō 6 (1578), is a naval engagement occupying a unique position in Japanese Sengoku history. In the first engagement, the Mōri fleet, wielding muskets and horokubiya (incendiary stone-fire arrows), annihilated the Oda naval forces. However, in the second engagement, the "iron-clad ships" (tetsukōsen) constructed under Kuki Yoshitaka's direction—operating under Nobunaga's orders—achieved their first combat action, achieving a one-sided victory over the Mōri fleet. These "iron-clad ships" are recognized in world naval history as among the earliest examples of armored vessels, exemplifying Nobunaga's genius in combining scientific innovation with strategic vision.

The Ishiyama Campaign and the Strategic Significance of Kizugawa Mouth—The Focal Point of a Ten-Year War

The Battle of Kizugawa Mouth occurred as part of Nobunaga's ten-year war against Ishiyama Hongan-ji (the Ishiyama Campaign, 1570-1580). Ishiyama Hongan-ji, situated where Osaka Castle now stands, exploited its strategic location facing Osaka Bay to receive supplies from the Mōri fleet while continuing its resistance. For Nobunaga, the key to capturing the temple lay in severing this supply line through Kizugawa Mouth.

The Ishiyama Hongan-ji's stubbornness stemmed from more than military strength alone. The faithful followers (monzō) gathered under the temple's high priest Kennyo, leading the Ikkō-ikki (Ikkō sect uprising), fought with conviction rooted in religious faith. The Jōdo Shinshū spirit of "whether advancing or retreating, both lead to Paradise" created a combat motivation fundamentally different from that of Sengoku warlords. Behind Nobunaga's requirement of ten years to capture the temple lay two factors: this religious commitment and the continued supply line maintained by the Mōri navy.

In the first Battle of Kizugawa Mouth in Tenshō 4 (1576), the Mōri fleet, centered on the Murakami kaizoku (Murakami pirates), commanded approximately 600 vessels that overwhelmed Nobunaga's naval force. The Mōri fleet's primary tactic involved combining horokubiya (incendiary clay spheres filled with gunpowder) with musket fire, and the predominantly wooden Oda vessels caught fire one after another. The Oda naval forces (under commanders like Manabe Sadatomo and Numazuma) suffered a catastrophic defeat, allowing supplies to reach Hongan-ji.

The Mōri Fleet—The Murakami Pirates' Technology and Tactics

The Murakami kaizoku represented a maritime military power that dominated the Seto Inland Sea. The three Murakami families—based in Innoshima, Noshima, and Kurushima—operated in coordinated networks as true professionals of the sea, possessing intimate knowledge of Seto Inland Sea geography. Their primary tactic involved "horokubiya fire attacks," using incendiary projectiles that proved devastating against wooden ships. Combined with long-range musket fire and close-quarters sword combat, their "naval warfare" techniques represented Japan's highest contemporary standard.

Regarding the swords of naval commanders, Seto Inland Sea warlords frequently favored blades crafted by smiths from the Chūgoku and Shikoku regions. Works by smiths from Bizen, Bingo, and Hōki provinces graced the swords of regional warlords. Among blades associated with the Mōri family, tachi in the Raishi and Bizen traditions are renowned; the Mōri lords Motonari and Terumoto treasured famous blades of these lineages.

Kuki Yoshitaka's Iron-Clad Ships—The Background of Technological Innovation and Its Connection to Sword Culture

Following the first defeat, Nobunaga ordered a fundamental revolution in naval warfare capability. The responsibility fell to Kuki Yoshitaka—a maritime daimyō based in Shima Province, later called a "pirate daimyō." Yoshitaka responded to Nobunaga's demands by directing the construction of an unprecedented large warship: the iron-clad ship.

According to the Oda Kōki (Oda's Chronicle), the iron-clad ship featured: a hull approximately 30 meters in length, armor plating of iron sheets covering the entire exterior, and capability to mount large-caliber muskets and cannons (kunigabushi), representing a revolutionary design. The concept of rendering wooden ships' incendiary attacks useless while achieving dominance through powerful cannon fire represented a naval design without contemporary parallel in the world.

The construction of the iron-clad ship possessed deep and fascinating connections to sword-forging techniques. Iron-plating technology developed primarily through "tatara" (traditional Japanese iron smelting), the very foundational technique that produced gyokuhagane (jewel steel)—the raw material of Japanese swords. The essence of Japan's metalworking techniques, refined through the sword industry via the traditional smelting of iron sand as raw material through charcoal reduction, was applied to the military technological innovation of the iron-clad ship.

The smelting and forging techniques that smiths from Bizen, Sōshū, and Yamashiro embedded in their blades elevated Japan's overall metalworking standards. The depth of the sword industry's capabilities enabled this technological leap in warship construction—a perspective crucial for understanding Sengoku-era Japanese technological history.

The Second Battle of Kizugawa Mouth and the Iron-Clad Ships' Overwhelming Victory

On the 6th of the 11th month in Tenshō 6 (1578), the second Battle of Kizugawa Mouth erupted. The Mōri fleet again attempted to transport supplies to Hongan-ji. However, the Oda fleet's formation presented a completely different picture. Leading the formation were six iron-clad ships commanded by Kuki Yoshitaka.

When the Mōri fleet unleashed their horokubiya, the iron-clad ships' armor repelled them. In response, the cannons mounted on the iron-clad ships successively shattered the Mōri wooden vessels. Mōri naval commander Murakami Takeyoshi resisted desperately, but faced with such an absolute technological advantage in armor, he could accomplish nothing. Many Mōri ships were sunk or set ablaze, and the remainder were forced to retreat. The second Battle of Kizugawa Mouth concluded with overwhelming Oda victory, completely severing the supply line to Hongan-ji.

As a result, cut off from supplies, Ishiyama Hongan-ji accepted a settlement with Nobunaga in Tenshō 8 (1580), with high priest Kennyo withdrawing from the temple. The ten-year Ishiyama Campaign reached its conclusion, with the technological victory at Kizugawa Mouth serving as the decisive factor.

Kobayakawa Takakage and Swords—The Cultural Legacy of the Defeated General

The command of the Mōri fleet in the second Kizugawa Mouth engagement fell to Kobayakawa Takakage (third son of Mōri Motonari). While renowned as a "master of stratagems," Takakage also possessed refined cultural sensibilities as a daimyō. He possessed deep expertise in swords and collected superior works in the Raishi and Bizen traditions. Portions of Kobayakawa Takakage's blade collection allegedly passed to Toyotomi Hideyoshi and Tokugawa Ieyasu after his ownership. When Kobayakawa Hideaki betrayed the Eastern forces at the Battle of Sekigahara (1600), the Kobayakawa clan was stripped of its holdings, yet the blades that constituted their legacy dispersed among warrior families and temples throughout Japan. Twenty-two years after their defeat at Kizugawa Mouth, the Kobayakawa's famous blades passed into the hands of former enemies—an ironic turn of history that demonstrates swords serve as "mirrors reflecting their owners' destinies."

"The sword is the soul of the warrior"—but equally, the sword served as an instrument of political and diplomatic communication during the Sengoku period. After the Mōri and Kobayakawa defeated in the naval battle withdrew, their famous blades continued to endure in Japanese history. This contrast—battlefield defeat coupled with the immortality of swords as cultural treasures—illuminates the historical significance of the Battle of Kizugawa Mouth from a richer perspective.

The Historical Significance of the Battle of Kizugawa Mouth—Nobunaga's Revolutionary Spirit and Japanese Technological History

The Battle of Kizugawa Mouth ranks among the most revolutionary naval engagements in Sengoku history. Within merely two years of the first defeat, Nobunaga constructed and deployed the "iron-clad ship," fundamentally transforming conventional naval warfare concepts. This approach—"learning from failure and overcoming it through technology"—exemplifies Nobunaga's essential character as an innovator.

The emergence of iron-clad ships, alongside the proliferation of firearms (introduction of muskets in 1543), represents a major inflection point in Japanese military technological history. The revolution combining gunpowder and iron symbolizes an era when the traditional katana's role on the battlefield was transforming. The iron plates used in the iron-clad ships that saw action at Kizugawa Mouth represented the pinnacle of contemporary Japanese iron-smelting technology, products of the metal-working traditions nurtured by the sword industry. Japanese swords and iron-clad ships—seemingly unrelated, yet bound together by "tatara smelting," Japan's distinctive iron culture.

Iron-Clad Ships and Swords—Two Technological Masterworks Born from Nobunaga's Revolutionary Vision

Oda Nobunaga's revolutionary spirit condensed into a single point: "destroying existing frameworks." The daring small-scale cavalry charge at the Battle of Inō, the surprise attack at Okehazama, the three-rank musket volleys at Nagashino (though disputed), and the iron-clad ships at Kizugawa Mouth—all represented the practical application of Nobunaga's methodology of "questioning convention and overcoming through technology."

This philosophy of "transforming warfare's conventions through technological innovation" resonated with an equally transformative period in Japanese sword history. During the Sengoku era, the uchigata-na (upright-worn sword) became widespread, displacing the tachi (downward-hanging blade) as the standard armament style. Where the tachi hung with blade-down in a suspended position, the uchigata-na was thrust blade-up through the sash, enabling faster draw-and-cut sequences. This "transition to the uchigata-na" itself constituted one technological innovation responsive to battlefield realities.

The "Heshikiri Hasebe" (Heshikiri Hasebe blade) favored by Nobunaga possessed qualities of a tanto and naginata, allegedly named after the anecdote that "Nobunaga cut down Matsunaga Hisahide before his dining table" ("heshikiri"). The "overwhelming through force" symbolized by this sword parallels the iron-clad ships' overwhelming of the Mōri fleet through brute power. In Nobunaga's era, swords and warships alike embodied the spirit of "iron's revolution."

The Seto Inland Sea Sword Culture—The Mōri and Kobayakawa Traditions and Their Aftermath

The sword culture of the Mōri and Kobayakawa families, defeated at Kizugawa Mouth, reflected deep traditions of the Chūgoku and Shikoku regions. Bizen Province (present-day Okayama Prefecture) stood as Japan's greatest sword-producing center, with the Osafune-centered blade-smith community maintaining vigorous activity from the Muromachi period onward. The works of master smiths—Kagekō, Nagamitsu, Kanemitsu, and others of the Bizen tradition—were treasured among western daimyō. Portions of Kobayakawa Takakage's blade collection allegedly passed to Toyotomi Hideyoshi and Tokugawa Ieyasu after his ownership. When Kobayakawa Hideaki betrayed the Eastern forces at the Battle of Sekigahara (1600), the Kobayakawa clan was stripped of its holdings, yet the blades that constituted their legacy dispersed among warrior families and temples throughout Japan. Twenty-two years after their defeat at Kizugawa Mouth, the Kobayakawa's famous blades passed into the hands of former enemies—an ironic turn of history that demonstrates swords serve as "mirrors reflecting their owners' destinies."