Choi Museon's Development of Cannons and the Victory at the Battle of Jinpo (Domestication of Gunpowder and Cannon Production in 1376, Battle of Jinpo in 1380) The late Goryeo period was a tumultuous era marked by devastating Wokou (Japanese pirate) invasions that ravaged the land and plunged the populace into misery. Coastal regions, in particular, were laid waste by Wokou plunder and slaughter, leaving the Goryeo court desperately searching for effective countermeasures. In this dire situation, Choi Museon keenly recognized the urgent need for new weaponry capable of destroying the Wokou's primary strength: their ships. He turned his attention to gunpowder weapons, which were then being introduced from China, and dedicated himself to developing indigenous gunpowder manufacturing technology. After years of relentless research and effort, Choi Museon finally succeeded in domesticating gunpowder production in 1376 (the 2nd year of King U's reign), by successfully combining saltpeter, sulfur, and charcoal. This marked a revolutionary turning point in strengthening Goryeo's national defense. Following this success, King U accepted Choi Museon's proposal and established the 'Hwatongdogam' (Bureau of Gunpowder Weapons), a national institution dedicated to the development of gunpowder weapons. Under Choi Museon's leadership, the Hwatongdogam developed and produced various types of cannons, such as the Daejanggunpo, Ijanggunpo, and Samjanggunpo, along with other gunpowder weapons like fire arrows (hwajeon) and explosive projectiles (jillyeopo). Goryeo thus acquired the independent capability to produce gunpowder weapons. The formidable power of the cannons developed by Choi Museon was fully demonstrated in the Battle of Jinpo, which took place in 1380 (the 6th year of King U's reign). At that time, the Wokou had assembled a massive fleet of over 500 ships and invaded Jinpo, located at the mouth of the Geum River, attempting to penetrate deep into the interior. The Goryeo navy, led by Na Se, Sim Deok-bu, and Choi Museon, sallied forth to Jinpo with warships equipped with these new cannons. The Goryeo forces concentrated their cannon fire on the densely packed Wokou ships, and the fire-spitting cannons quickly turned the Wokou fleet into a blazing inferno. Most of the Wokou ships were either sunk or burned, and the pirates suffered immense casualties before retreating. This battle was a monumental event in Korean history, marking the first successful large-scale naval engagement won through the use of cannons. The victory at the Battle of Jinpo became a decisive turning point in Goryeo's struggle against the Wokou. The Wokou could no longer freely invade Goryeo's coasts, and this victory significantly influenced subsequent land operations to eradicate the pirates. Furthermore, Choi Museon's achievements in domesticating gunpowder and developing cannon technology were inherited and further advanced during the Joseon Dynasty, forming a crucial foundation for strengthening national defense. His legacy is celebrated not merely as a technological breakthrough but as a monumental historical contribution that laid the groundwork for self-reliant defense and protected the lives of the people.