Episode Five of the IMJIN WAR, based on my book “The Imjin War: Japan’s Sixteenth-Century Invasion of Korea and Attempt to Conquer China.” As we saw in the previous two episodes, the musket played a huge role in the initial success of Hideyoshi’s invasion of Korea. The spearhead contingents of his invasion army, armed with thousands of matchlock muskets, swept north to take Seoul in only three weeks.