From A History of the Modern Chinese Army, by Xiaobing Li (U. Press of Kentucky, 2007), pp. 205-206 (footnote references omitted):
SINCE THE FOUNDING OF THE PRC in 1949, China has involved itself in two wars in Vietnam. During the French Indochina War (the First Indochina War), from 1949 to 1954, it assisted the People’s Army of Vietnam (PAVN) against French forces. China sought to secure its southwestern border by eliminating the Western power’s presence in Vietnam. The PLA’s military assistance to Vietnam maintained Beijing’s brooding influence in Vietnam, Laos, and Cambodia throughout the Cold War. The PLA’s second involvement occurred from 1965 to 1970, when China sent 320,000 troops to aid North Vietnam against American forces in the Vietnam War (the Second Indochina War). Through its war efforts in North Vietnam, Beijing tried to break a perceived U.S. encirclement of China. But China was not interested in a “more powerful” Vietnam on its southern border. Some Vietnamese Communists complained about China’s limited assistance to the Viet Minh.
This chapter traces the rise and fall of the Sino-Vietnamese alliance through the two episodes of Chinese involvement in Vietnam. It examines the changing international strategic environment and external conflicts that influenced the Chinese military’s organization and strategy. It begins with Mao’s continuous revolution, his central theme in shaping Chinese foreign policy and security strategy. The CCP supported Ho Chi Minh, the leader of North Vietnam, in his war against the French forces in 1946-54. The stories of Senior General Chen Geng and General Wei Guoqing show that Chinese economic and military aid to Ho and the PAVN increased until the end of the French Indochina War. The PLA continued to support Ho’s regime against the U.S. Air Force and Navy in the Vietnam War in 1965-70. The PLA’s deployment successfully deterred any U.S. invasion of North Vietnam, as the United States feared provoking China…. In 1968, Chinese influence over North Vietnam decreased as Soviet influence grew. The PLA withdrew its antiaircraft artillery units in March 1969 and its support troops by July 1970.
The 1960s was the most controversial as well as the most crucial decade in Chinese military history. By 1969, the Soviet Union had replaced the United States as Beijing’s leading security concern, prompting changes in China’s strategic thought. Thereafter, the high command prepared to repel a Soviet invasion. In 1969-71, the PLA clashed with the Soviet forces along the Sino-Soviet border. As a result of its frequent engagements, the PLA increased to more than six million men, the highest point in its history. The Soviet threat and conflicts pushed the Chinese leaders to improve their relations with the United States. Their strategic needs eventually led to the normalization of the Sino-American relationship in the early 1970s.