The correct answer is: "North Korea invaded South Korea".
The Korean War was a conflict that took place between 1950 and 1953 and confronted, on one side, North Korea that enjoyed the support of the URSS and China, and South Korea with the main assitance of the United States. It is considered a proxy war within the larger conflict known as Cold War.
The conflict started the 25th of June of 1950, when after some troubles in the border, North Korea started an invasion on South Korea. Both territories claimed to represent the sole legitimate goverment of Korea. A communist state was established in the North while a capitalist one emerged in the South.
