In June of 1950, the North Korean army invaded South Korea. The United States led a United Nations force into the Korean Peninsula to halt the aggression. The forces of several countries joined the effort to thwart this naked aggression of the North Koreans (supported by the Soviets and China). Due to the daring and masterful tactics of General MacArthur, the North Koreans were turning tail and heading toward the Chinese border. It is here that the Chinese decided to interfere in this United Nations action.
The Chinese entry into the war was long planned. Even as early as August, 1950, the decision was made for Chinese entry into the war. However, at that time, it wasn't deemed necessary as the North Koreans were winning the war. However, within a few months, the situation changed dramatically. The decision was made on October 2 to enter the war by crossing the Yalu on October 15. While the crossing was actually made on the 16th, it was on this day that the Chinese openly opposed a United Nations Security Council Resolution by crossing the Yalu River and engaging the U.S.-led multinational forces.
What followed was a three-year long bloodbath, resulting in the continuation of the brutal regime in North Korea that started the war in the first place. You have to hand it to China, they constantly defend and support the world's most despotic regimes.
In light of the Korean War, I find it amusing that the Chinese were trying to use the same Security Council they ignored to defend another despot, Saddam Hussein.