U.S. #921 is part of the Overrun Countries Series, which honors each of the nations invaded by Axis powers during World War II. It pictures the flag of Korea, which features the Chinese Yin and Yang symbol and four symbols representing the Taoist philosophical ideas of the universe – harmony, symmetry, balance, and circulation. Korea and World War II Prior to World War II, Korea had already been under Japanese control for decades. In 1939, more than five million Koreans were forced into labor and tens of thousands more were forced into the Japanese military. During the Japanese occupation, traditional Korean culture was discouraged, including the use of the Korean language. After the war, Korea was split at the 38th parallel, with the Soviet Union administering the North and the U.S. administering the South.