Excerpts from Books and Wikipedia
"The scale of the offensive, though, was clear evidence of a long-planned campaign to reunify Korea. The original division in 1945 had been intended as a temporary expedient until the United States and the Soviet Union could agree on the future of the country. . . . The Soviet Union was boycotting the UN Security Council because the Americans had refused to give the communist regime a permanent seat on the body. Because of this soviet abstention, and thus their inability to use their power of veto, Washington was able to get Security Council resolutions passed condemning the North Korean invasion and calling on member states to send forces to defeat it. With varying degrees of reluctance and motivated more by their dependence on the United States than by enthusiasm for Korea, 15 nations contributed forces. Five other sent medical units to a US-led coalition, only nominally under the UN flag, fighting the first open conflict of the Cold War." [Furtado: 1001 Days]