The Soviet Union was impelling the expansion of communism. Following WWII, agreements between powers divided Korea into two parts with the Soviets in the north and the US in the south, all in advisory capacity. Eventually the two nations might vote for unification under the UN auspices.
The UN was established in 1945.
Under the urging of the Soviets, the North Koreans invaded the south to force reunification at a time when they though the US was unlikely to intervene. In the diplomatic jockeying at the UN assembly, one ploy or another caused the Soviets to walk out temporarily and the assembly was able to vote for an intervention.
Of course the US provided the bulk of the military forces, but other nations, including the UK, Australia and more also participated.
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