North Korea invaded South Korea on June 25, 1950.
The same day the United Nations Security Council adopted Resolution 82, calling on North Korea to cease hostilities and withdraw to the 38th parallel.
On June 27 1950, the Security Council adopted Resolution 83, recommending that members of the United Nations provide assistance to the Republic of Korea "to repel the armed attack and to restore international peace and security to the area".
On July 7 1950, the Security Council adopted Resolution 84, recommending that members provide military forces and other assistance to South Korea and "make such forces and other assistance available to a unified command under the United States of America".
(Links to the resolutions to be found
here.)
Apparently the possibility exists for the UN to threaten military action.