Why did China enter the Korean War? What made Mao Zedong, only one year after the establishment of the People's Republic of China, decide to assist North Korea in fighting a coalition of nearly all of the Western industrial powers? Many scholars have traditionally regarded Beijing's decision to enter the war as a reaction to Washington's mistake in crossing the 38th Parallel and marching towards the Yalu. Based on extensive use of newly available Chinese sources, however, this study argues that China's entry into the Korean War was brought about by concerns much more complex than simply safeguarding the Chinese-Korean border. The author places the making of the Sino-American collision in Korea in the broader context of both the continuous development of the Chinese Communist Revolution after its nationwide victory and the emerging Cold War. He provides a fresh analytical framework within which to understand the foreign policy and security strategy of the People's Republic of China.