The Japanese invasion of Korea began three weeks earlier, when 400 transports landed 18,700 soldiers under Konishi Yukinaga at Busan on 23 May 1592. Over the following weeks, Toyotomi Hideyoshi's full invasion force of 158,000 men poured across the strait. The Japanese armies swept north with terrifying speed, seizing the Korean capital of Hanseong by mid-June. Korea's regional naval commanders panicked. Gyeongsang Left Navy Commander Bak Hong scuttled his entire fleet of 100 warships without firing a shot. Gyeongsang Right Navy Commander Won Kyun, mistaking fishing boats for the enemy, destroyed his own weapons and stores before his subordinates talked him out of deserting -- by which time he had only four ships left. When Won Kyun begged for help, Yi Sun-sin could not legally leave his jurisdiction without orders. But he used the delay wisely, acquiring navigational charts of the treacherous coastal waters and executing deserters to stiffen discipline.

Yi finally sailed with 39 warships and 46 smaller boats, incorporating Won Kyun's surviving four vessels. When scouts reported Japanese ships at Okpo, Yi arranged his fleet in a line formation -- heavy warships in the center, lighter craft on the flanks, Won Kyun trailing cautiously at the rear. They entered the harbor to find the Japanese transports mostly unmanned, their crews busy plundering the Korean village ashore. Yi attacked immediately. By the time the Japanese noticed the Korean fleet through the harbor smoke, they were already surrounded and hemmed into the port. The Japanese scrambled aboard their ships and tried to fight back with arquebuses, but the firearms that devastated infantry on land could not penetrate the thick wooden hulls of Korean warships. Yi's cannon and fire arrows shattered one transport after another. Soldiers threw their weapons and armor overboard, abandoned their ships, and leaped into the water. Twenty-six transports were destroyed before Yi, resisting the urge to land troops and chase survivors on the island, withdrew to preserve his fleet's strength.

The victory at Okpo was only the beginning of a furious two-day sweep. That same afternoon, Yi received word of five more Japanese ships nearby and gave chase. The Japanese fled to Happo harbor and abandoned their vessels, which Yi ordered burned. The following morning brought news of thirteen more transports at Jeokjinpo, where -- once again -- the Japanese were looting and burning a Korean village. Yi destroyed eleven of the thirteen ships. Flushed with success, Yi and Won Kyun briefly considered pressing on to Busan to strike the main Japanese fleet. But word arrived of Hanseong's fall eight days earlier, and the admirals decided discretion was the wiser course. Yi returned to his base at Yeosu and wrote a detailed report to King Seonjo, noting with bemusement the strange appearance of samurai helmets found aboard the wrecked ships.

The Battle of Okpo was small in scale compared to what would follow, but its consequences were enormous. The destruction of Japanese transports along the southern coast created anxiety in the Japanese command about the security of their supply lines -- a vulnerability Yi would exploit ruthlessly in the months ahead. At the Battle of Sacheon shortly afterward, he would deploy his famous turtle ships for the first time, armored vessels with spiked roofs that became the terror of the Japanese navy. What began in the smoky harbor of Okpo on a June morning grew into one of history's most remarkable naval campaigns, led by an admiral who combined tactical brilliance with a deep emotional commitment to protecting the Korean people from the invaders ravaging their coast. Today a commemorative monument at Okpo harbor marks where Yi Sun-sin's legend was born.