After World War II Korea got divided into Northern Communist state backed by U.R.S.S. and a Southern Capitalist state supported by U.S.A.
The Korean War started on June 1950 when North Korea supported by the Soviet Union crossed the 38th Parallel and invaded South Korea.
75,000 North Korean soldiers moved across the boundary and overran South Korea; thus North Koreans took control of of Seoul; thereby (d).