On May 23, 1592, Toyotomi Hideyoshi (豊臣秀吉, Toyotomi Hideyoshi) launched the Imjin War (壬辰倭乱, Imjin Waeran) — the Japanese invasion of Korea — with one of the largest amphibious forces in the history of East Asia. His commanders, including Konishi Yukinaga (小西行長, Konishi Yukinaga) and Kato Kiyomasa (加藤清正, Kato Kiyomasa), moved through Korea at devastating speed. Seoul fell within weeks. The Korean army, unequipped and unprepared for the ferocity of battle-hardened Japanese samurai armed with arquebuses, collapsed in front of them. Hideyoshi's vision was a land bridge from Japan through Korea to China. The sea, his planners assumed, would be even easier than the land.

They had not yet encountered Yi Sun-sin (李舜臣, Yi Sun-sin).

Yi had been born on April 28, 1545, and had spent his career in land-based military posts before being appointed to command the Left Jeolla Naval District in early 1592 — just months before the invasion began. He had never commanded a naval battle in his life. He had, however, spent those months building and drilling his fleet with meticulous care: 24 large board-roofed panokseon (板屋船, panokseon) warships, 15 smaller hyeopson (협선, hyeopson), and 46 support vessels. When the invasion came, he moved.

To the east, the Korean naval situation was already catastrophic. Admiral Won Kyun (元均, Won Kyun) had been overwhelmed by the invasion's opening naval attacks and reduced to just 4 surviving ships. Yi linked up with Won Kyun's remnants on June 15, combining their forces into approximately 91 vessels total, and continued west and south. On June 16, 1592, Yi's fleet arrived at Okpo (玉浦, Okpo) harbor, off Geoje Island (巨済島, Geoje-do) on Korea's southern coast.

What Yi found at anchor exceeded expectations. More than 50 Japanese transport ships sat in the harbor — and most of their crews were ashore, scattered through the coastal village above the bay, loading their arms with plunder. The soldiers who had shattered Korean infantry on land were at that moment carrying stolen goods up a hillside road. They looked up and saw the Korean fleet already inside the harbor, guns pointing at their ships.

Yi encircled the Japanese vessels and opened fire with ship-mounted cannons. The Japanese ran for their boats and grabbed their arquebuses (火縄銃, hinawaju) — the weapons that had been lethal against Korean infantry on land. Against the thick-planked hulls of the Korean panokseon, the bullets rattled off like pebbles. The Japanese ships were lighter and faster but not built for this kind of fight. The Korean cannons tore through them. Twenty-six Japanese ships were destroyed. Not a single Korean vessel was lost. Yi then moved on; he did not pursue fleeing Japanese sailors into the village. His war was against ships, not men, and supply lines, not glory.

The same day saw further engagements at Happo (합포, Happo); on June 17, the fleet continued to the Battle of Jeokjinpo (적진포 해전, Jeokjinpo Haejeon). The entire first naval campaign of 1592 was a string of unbroken Korean victories. It was Yi's first naval battle. It would not be his last — and he would not lose one in 23 battles across the entire war.

The strategic consequences became clear within weeks. Yi's victories began severing Japan's western supply line — the sea route that was supposed to feed Hideyoshi's northern armies pushing toward China. Without reliable resupply, the land advance began to stall. Hideyoshi had sent his army to conquer a peninsula; Yi Sun-sin was in the process of turning the sea into a wall.