In the latter half of the 14th century, Koryo (918–1392) had to fight against the intruding Japanese pirates in a difficult situation.
The Japanese pirates intruded into the country on a large scale, and they went so far as to make inroads into its coast and inland, running amok to plunder all the cargo ships carrying cereal requisitioned as agricultural tax in kind towards Kaegyong (the capital city of Koryo).
All the places where the pirates had once set their feet were turned into ruin, and as waterways were blocked by them, the granaries in Kaegyong became empty.
Such circumstances demanded an urgent measure be taken to annihilate the pirates not only on land but also on the sea. The relevant measure was to build a powerful fleet of vessels. But the short-sighted ministers in the government would not approve it.
At this juncture, there was a person who invented gunpowder and powder weapons—he was Choe Mu Son.
As he grew up, witnessing the damages and misfortunes his country and people suffered from Japan’s intrusion, he thought how to wipe out the enemy.
Fire-attack tactics should be employed to burn down all the enemy vessels in order to destroy the pirates intruding from the sea; then, the enemy devoid of any means would not intrude again while those landed on the shore would be in a fine fix, he thought.
He first buckled down to the research into the method of making niter to obtain gunpowder.
With skilled artisans, he arranged experimental and manufacturing equipment, working on experiment day after day. In October 1373, he made niter at last, and succeeded in developing the method of making gunpowder.
Immediately, he reported to the government the advantages of gunpowder and, at the same time, suggested defeating the Japanese pirates by making gunpowder and powder weapons. But some incompetent ministers in the government rejected his proposal.
He could persuade them only after demonstrating the power of gunpowder.
At that time, General Choe Yong actively helped him in making powder weapons and building vessels to be equipped with those weapons.
With his help, Choe Mu Son made different cannons such as taejanggun and ijanggun, various shells like hwajon, cholryongjon, phiryongjon, cholthanja, ryuhwa and juhwa and arrows with fire wicks. And he built over 130 vessels which could be equipped with those weapons and deployed them at the estuary of a river.
In 1380, Choe Yong became the navy commander of Koryo, and was determined to make a trial of the vessels equipped with powder weapons in actual warfare.
At that time, the Japanese pirates on over 500 vessels landed on the mouth of the Kum River, and attacked several naval bases in Jolla Province. Informed of this, 100 vessels of the Koryo navy set sail.
When the fleet approached Jinpho, the destination, Choe Mu Son wore an unusually serious look; the time when the powder weapons he had made through painstaking efforts for years was coming nearer.
The Koryo fleet sailed towards the enemy vessels. The Japanese pirates, with several fleets, manoeuvred to encircle the Koryo fleet.
At this juncture, the Korean fleet fired on the enemy all at once; shells like cholthanja and cholryongjon destroyed masts and decks of the enemy vessels and hwajon and juhwa on fire hit the targets.
Most of the enemy soldiers were burned or drowned to death. Seeing the serious situation, the enemy veered their ships to flee.
The Korean fleet launched an all-out offensive, and the courageous seamen jumped on the enemy vessels, killing the Japanese pirates mercilessly.
In this way, 500 enemy vessels were sunk or burned by the powerful powder weapons of the Koryo fleet.
This is the famous naval battle off Jinpho, where the Koryo fleet annihilated the Japanese pirates by using powder weapons for the first time.