Imagawa Yoshimoto controlled three provinces -- Suruga, Totomi, and Mikawa -- and in the spring of 1560 he assembled an army of 25,000 to march along the Tokaido highway toward Kyoto. His aim was to challenge the weakening Ashikaga shogunate for dominance over Japan. The route took him directly through Owari Province, recently unified under Nobunaga, a young lord not yet taken seriously by the established powers. The Imagawa forces swept through Nobunaga's border defenses with ease. Matsudaira Motoyasu, a vassal of Yoshimoto who would later become Tokugawa Ieyasu, captured Marune fortress. Washizu fell as well. Yoshimoto made camp at a place called Dengakuhazama, in the village of Okehazama, just outside what is now the city of Nagoya. His generals were celebrating. The conquest of Owari appeared to be already settled.

Inside Kiyosu Castle, Nobunaga's generals debated whether to fortify the castle and withstand a siege. Nobunaga dismissed the idea -- Kiyosu could not hold against 25,000. "Only a strong offensive policy could make up for the superior numbers of the enemy," he declared. His scouts knew the terrain around Okehazama intimately; Nobunaga had conducted war games there for years, disguised as falconry hunts. That night, he told his men that waiting meant death and sent them home to rest. Before dawn, he prayed at Atsuta Shrine, then led his small force to a fortified temple called Zensho-ji, positioning them within striking distance of the Imagawa camp. To deceive enemy scouts, he ordered banners and flags planted around the temple to suggest a much larger army.

The afternoon of June 12 was brutally hot. Inside the Imagawa camp, soldiers celebrated their easy victories. Many had stripped off their armor. Then the sky darkened. A violent thunderstorm rolled across the narrow valley where Yoshimoto had pitched camp, sheets of rain hammering the tents and drowning out all other sound. Under cover of the downpour, Nobunaga's men struck. They hit the heart of the camp, not the flanks -- a frontal assault that exploited surprise, terrain, and weather. Panic erupted. The Imagawa soldiers, many unarmed and disoriented, broke ranks and fled. Yoshimoto himself fought. When the Oda samurai Mori Shinsuke attacked with a spear, Yoshimoto sliced through the weapon's shaft and cut into Shinsuke's knee. But a second warrior, Hattori Koheita, tackled the general and took his head. With Yoshimoto and nearly all his senior officers dead, the massive army collapsed.

The Battle of Okehazama is counted among the most consequential events in Japanese history. The Imagawa clan, stripped of its leader and its prestige, disintegrated within years. Matsudaira Motoyasu, freed from his obligations as an Imagawa vassal, allied with Nobunaga and eventually became Tokugawa Ieyasu, founder of the Tokugawa shogunate that would rule Japan for over 250 years. Among Nobunaga's own retainers, the battle marked the first time he noticed a lowly sandal-bearer named Kinoshita Tokichiro, who would rise to become Toyotomi Hideyoshi, the second of Japan's three great unifiers. One afternoon's gamble in a thunderstorm set all three men on the path that would end the Sengoku period and reunify the nation.

Today the battlefield is a quiet park in Toyoake City, just south of Nagoya. In 1937, Japan's Ministry of Education designated the site as a National Historic Site. Seven granite pillars stand in a row at Minamiyakata, each one representing one of Yoshimoto's seven warlords. The first pillar bears the inscription: "Imagawa Yoshimoto was killed here." The narrow valley where 25,000 troops once camped is now suburban neighborhood, the Tokaido highway replaced by modern roads. But the geography still reads clearly -- the confined space, the surrounding hills, the terrain that favored a smaller force willing to take an outrageous risk. Standing among the pillars, it is easy to picture the rain falling and the banners going down.