The United States Army Military Government in Korea, also known as USAMGIK, was the official ruling body of the southern half of the Korean Peninsula from September 8, 1945 to August 15, 1948. Many of the foundations for the modern South Korean system were laid during this period.

The country in this period was plagued by political and economic chaos, which arose from a variety of causes. The aftereffects of the Japanese occupation were still felt in the occupation zone, as well as in the Soviet zone in the North. Popular discontent stemmed from the U.S. Military Government's support of the Japanese colonial government; then once removed, keeping the former Japanese governors on as advisors; by ignoring the functional and popular People's Republic of Korea (PRK); by censoring and forcibly disbanding the PRK; and finally by supporting United Nations elections that divided the country.

In addition, the U.S. military was largely unprepared for the challenge of administering the country, arriving with no knowledge of the language or political situation. Thus, many of their policies had unintended destabilizing effects. Waves of refugees from North Korea (estimated at 400,000) and returnees from abroad also helped to keep the country in turmoil.