By the mid-14th century, Korean leaders recognized the need for firearms to counter frequent Japanese pirate raids. China supplied gunpowder and weapons in 1374, but production in Korea only began after Ch'oe Musŏn learned to refine potassium nitrate from Chinese merchants around 1374–1376. Appointed head of a newly formed government office for gunpowder production in 1377, Ch'oe oversaw the development of rockets such as the juhwa and other early firearms. Building on these advances, Korean engineers developed the hwacha in 1409 during the Joseon Dynasty, with contributions from Yi To and Ch’oe Haesan. The hwacha was a wooden cart fitted with launch modules that could fire up to 200 rocket-propelled singijeon arrows or dozens of iron-tipped bolts from gun barrels. A major improvement came in 1451 under King Munjong, who produced a versatile version capable of firing either 100 rockets or 200 projectiles at once. By year’s end, hundreds were deployed across the peninsula, including 50 in Hanseong and 80 along the northern frontier. Variants such as the Mangam Hwacha, decorated with painted dokkaebi faces, could unleash 600 bullets from 40 small cannons. The hwacha’s greatest test came during the Japanese invasions of 1592–1598. Stationed in fortresses, hwachas were used defensively against tightly packed samurai formations. At the Battle of Haengju, 3,400 Koreans equipped with 40 hwachas repelled 30,000 Japanese troops, inflicting heavy losses and securing one of the most famous defensive victories of the war.