Answer:
The action occurred in June 1950 that caused the United Nations to respond with military action was the North Korean invasion of South Korea.
Explanation:
The Korean War took place between 1950 and 1953. Its components were the Republic of Korea (or South Korea), supported by the armed forces of several countries commanded by the United States; and the Democratic People's Republic of Korea (or North Korea), supported by the People's Republic of China and the Soviet Union. The war was one of the earliest episodes of the Cold War.
Five years before, after the end of World War II, the United States and the Soviet Union agreed to split Korea into two. They drew the border on the 38th parallel, leaving the North in charge of the Soviet Union and the South in charge of the United States. Each superpower controlled in its respective area the constitution of two new states that were under their respective orbits: the Democratic People's Republic of Korea in the north and the Republic of Korea in the south.
Although negotiations were held for the reunification of Korea in the months before the war, the tension intensified with cross-border skirmishes and incursions on the 38th parallel. The escalation of tension degenerated into an open war when North Korea invaded South Korea on June 25, 1950
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