What was the first invasion of Korea?

The Romans invaded North Africa, then Britain, then Gaul, then Germania, then Dacia, then Greece, then Egypt, and then the middle east. The Mongols invaded Central Asia, then China, Siberia, Eastern Europe, and the Middle East. The Chinese invaded Korea, Vietnam, Mongolia, Tibet, Myanmar, and others. The Japanese invaded Korea, China, and Taiwan.<br><br>But who was the first to invade Korea? Did Korea even exist as a state before the Gojoseon?<br><br><br>EDIT: I know the Japanese invaded Indonesia and Malaysia, and the Chinese invaded Afghanistan, but I didn't want to make my comment too long.