By May 1592, Toyotomi Hideyoshi (豊臣秀吉, Toyotomi Hideyoshi) had achieved something no one in living memory had managed: the unification of Japan. After a century of civil war among competing warlords, he had subdued or co-opted every major power on the islands. He was 55 years old and in command of the most battle-hardened military in Asia. The problem — though he may not have seen it as one — was that those armies had nowhere left to go.

Hideyoshi's answer was the continent. His stated ambition was the conquest of Ming China; Korea was simply the road that led there. He wrote to the Korean king demanding safe passage for his forces. When the Koreans refused and sent back evasive replies over several years, he launched the invasion. According to some accounts, he told his advisors he would conquer India within ten years. Whether he meant it literally or not, the army he sent north on May 23, 1592 was an instrument calibrated for continental warfare.

The invasion force totaled approximately 158,000 men in its first wave — the largest Japanese army ever deployed outside Japan. It was organized into divisions, and the honor of leading the vanguard went to Konishi Yukinaga (小西行長, Konishi Yukinaga), commander of the First Division, with approximately 7,000 soldiers. Konishi was a striking choice. He was a Christian daimyo, having converted to Catholicism and been baptized under the name Dom Agostinho. He was chosen reportedly more for his diplomatic abilities than his battlefield record, and some accounts suggest he was opposed to the invasion. He obeyed anyway.

On the morning of May 23, 1592, Konishi's division departed Tsushima Island (対馬, Tsushima) — the stepping stone between Japan and Korea that the So (宗, So) clan had managed for generations as a trade intermediary — and crossed the Korea Strait. The fleet arrived at Busan harbor by evening. On the Korean side, lookouts spotted the ships but the naval commander Won Kyun initially dismissed them as Japanese trading vessels. A second report — of over a hundred more ships on the horizon — finally raised the alarm. By then, Konishi's men were already in the harbor.

The Korean peninsula had been at peace for so long that its military readiness had severely deteriorated. Korean commanders had argued for years about the reliability of intelligence suggesting a Japanese invasion was coming; the warnings were discounted. The arquebus (火縄銃, hinawaju) — introduced to Japan by Portuguese traders in 1543, nearly fifty years before the invasion — was a weapon Korean defenders had little experience countering. Japanese soldiers were among the most experienced gunmen in the world, hardened by decades of civil war in which firearms had become decisive.

The attack on Busan's outer forts began before dawn on May 24. Korean commander Jeong Bal (鄭撥, Jeong Bal), stationed at the Busanjin (釜山鎮, Busanjin) fortress, was offered the chance to stand aside. He refused. By 9 o'clock in the morning he was dead, shot by an arquebus. The fort fell within hours. Kato Kiyomasa (加藤清正, Kato Kiyomasa) with the Second Division (22,000 men) and Kuroda Nagamasa (黒田長政, Kuroda Nagamasa) with the Third Division followed in the days after. The full scale of the invasion became clear.

Seoul fell on June 12, 1592 — less than three weeks after the first Japanese soldier set foot on Korean soil. Korean King Seonjo (宣祖, Seonjo) fled north toward the Yalu River. Pyongyang fell on July 20. Japanese forces reached nearly to the Chinese border. The conquest of China seemed, briefly, possible.

Then the sea turned against Japan. Admiral Yi Sun-sin (李舜臣, Yi Sun-sin) of the Korean navy began destroying Japanese supply convoys with his geobukseon (거북선), the armored warships known as turtle ships — repeatedly defeating Japanese naval forces and severing the supply lines that the overextended land army desperately needed. Ming China sent intervention forces. Korean guerrilla resistance spread across occupied territory. A war that had begun with the speed of a landslide settled into six years of attrition. Hideyoshi died in September 1598; Japan withdrew. The invasion gained nothing. Korea was devastated for a generation. Japan's finances were severely strained. And the Toyotomi clan, which had launched the greatest foreign expedition in Japanese history, emerged weakened — fatally so, as Tokugawa Ieyasu would demonstrate two years later at Sekigahara (関ヶ原, Sekigahara).