The Korean commander Won Gyun had seen Japanese trading ships before. When 400 vessels appeared on the southern horizon in the spring of 1592, he hesitated. By the time he understood what he was looking at, the beach at Busan was already churning with men.

Konishi Yukinaga was not the samurai you might expect at the head of history's largest amphibious assault on the Asian continent. His family were wealthy merchants from Sakai — traders, not warriors. He had converted to Christianity sometime in the 1580s, taking the baptismal name Agostinho (Dom Agostinho). He commanded the First Division of Toyotomi Hideyoshi's invasion force: approximately 18,700 men aboard 400 transports, crossing from the island of Tsushima to the southeastern tip of the Korean peninsula. The crossing took a single day.

Hideyoshi's plan had a dreamlike simplicity. He had unified Japan after a century of civil war — the 戦国時代 (Sengoku Jidai), the Age of Warring States — and he saw no reason to stop at the water's edge. Korea was a stepping stone. The real prize was China, the Ming dynasty, the Middle Kingdom itself. He had told his generals to plan for a campaign that would end in Beijing.

What made the opening of this campaign so stunning was the technology. Roughly a quarter of the 158,000-man invasion force carried 鉄砲 (teppō), matchlock muskets introduced to Japan by Portuguese traders at Tanegashima Island in 1543. In forty-nine years, Japanese armorers had refined the weapon and Japanese commanders had developed volley-fire tactics that no army in East Asia had encountered in mass formation. Korean infantry, trained with bow and spear for a century of relative peace, were outranged by 500 meters. The psychological shock was total.

Busan fell in hours. The fort's garrison fought, but the outcome was never in doubt. Konishi's division drove north along the central corridor through the Korean peninsula while his rival Kato Kiyomasa raced up the eastern route in a parallel thrust — the two commanders competing as much with each other as with the enemy. Konishi arrived at the Joseon capital Hanseong (present-day Seoul) to find the city already abandoned; King Seonjo had fled north toward the Chinese border three days before. Konishi kept moving, chasing the king toward Pyongyang.

Hideyoshi, watching from his vast forward headquarters at Nagoya Castle in Hizen — a purpose-built fortress in northern Kyushu, never used again after the invasion — declared the campaign proceeding exactly as planned. He began drafting a letter to the Ming emperor announcing the imminent arrival of Japanese forces.

He never sent it. Out at sea, a different story was being written.

Admiral Yi Sun-sin (이순신), the Right Naval Commander of Jeolla Province, had spent years preparing for an invasion that the Joseon court refused to believe was coming. His 거북선 (geobukseon, "turtle ships") — iron-clad vessels bristling with cannon — began hunting Japanese supply convoys along the Korean coast. The Japanese army advanced inland on the assumption that its supply lines behind it were secure. They were not. Yi Sun-sin destroyed convoy after convoy. The brilliant three-week land advance, which had pushed from Busan to Pyongyang in less than sixty days, stalled. Then the Ming dynasty sent 50,000 soldiers across the Yalu River, and the Japanese advance reversed entirely.

A ceasefire in 1593 produced four years of negotiations that both sides entered in bad faith. Hideyoshi's demands — territory, tribute, a Ming princess for the Japanese emperor — were presented to Chinese envoys in a form so altered from what the Ming court actually received that no agreement was possible. When talks finally broke down, Hideyoshi launched a second invasion in 1597. It failed faster than the first. He died in 1598, still insisting on victory, and his generals quietly withdrew.

The war consumed what Hideyoshi had built. His treasury was drained. His top commanders had spent six years fighting each other — Konishi and Kato barely spoke by the campaign's end — rather than building the domestic alliances that might have kept the Toyotomi family in power. Tokugawa Ieyasu, who had preserved his eastern domains and kept most of his forces out of Korea, emerged from the 1590s stronger than anyone. He would finish what Hideyoshi started at the Battle of Sekigahara in 1600.

Konishi Yukinaga, the Christian general who had crossed the sea with guns brought by Western missionaries, fought on the losing side at Sekigahara. Captured, he refused to commit ritual suicide — his faith forbade it — and was publicly beheaded in Kyoto.

Hideyoshi, who had looked at a continent and thought: why stop here? Never crossed the water to see what his ambition had done.