In 1950, the People's Republic of China extended diplomatic recognition to the Viet Minh's Democratic Republic of Vietnam and sent weapons, as well as military advisors led by Luo Guibo to assist the Viet Minh in its war with the French. The first draft of the 1954 Geneva Accords was negotiated by French Prime Minister Pierre Mendès France and Chinese Premier Zhou Enlai who, fearing U.S. intervention, urged the Viet Minh to accept a partition at the 17th parallel.
China's ability to aid the Viet Minh declined when Soviet aid to China was reduced following the end of the Korean War in 1953. Moreover, a divided Vietnam posed less of a threat to China. China provided material and technical support to the Vietnamese communists worth hundreds of millions of dollars. Chinese-supplied rice allowed North Vietnam to pull military-age men from the paddies and imposed a universal draft beginning in 1960.
In the summer of 1962, Mao Zedong agreed to supply Hanoi with 90,000 rifles and guns free of charge. Starting in 1965, China sent anti-aircraft units and engineering battalions to North Vietnam to repair the damage caused by American bombing, rebuild roads and railroads, and to perform other engineering works. This freed North Vietnamese army units for combat in the South.
Sino-Soviet relations soured after the Soviets invaded Czechoslovakia in August 1968. In October, the Chinese demanded North Vietnam cut relations with Moscow, but Hanoi refused. The Chinese began to withdraw in November 1968 in preparation for a clash with the Soviets, which occurred at Zhenbao Island in March 1969. The Chinese also began financing the Khmer Rouge as a counterweight to the Vietnamese communists at this time. China's withdrawal from Vietnam was completed in July 1970. The Khmer Rouge launched ferocious raids into Vietnam in 1975–1978. Vietnam responded with an invasion that toppled the Khmer Rouge. In response, China launched a brief, punitive invasion of Vietnam in 1979.
On the anti-communist side, South Korea had the second-largest contingent of foreign troops in South Vietnam after the United States. The first South Korean troops began arriving in 1964 and large combat battalions began arriving a year later, with the South Koreans soon developing a reputation for effectiveness. Indeed arguably, they conducted counterinsurgency operations so well that American commanders felt that Korean area of responsibility was the safest.
Approximately 320,000 South Korean soldiers were sent to Vietnam, each serving a one year tour of duty. Maximum troop levels peaked at 50,000 in 1968, however all were withdrawn by 1973. About 5,000 South Koreans were killed and 11,000 were injured during the war.
Australia and New Zealand, close allies of the United States and members of the Southeast Asia Treaty Organization (SEATO) and the ANZUS military co-operation treaty, sent ground troops to Vietnam. Both nations had gained experience in counterinsurgency and jungle warfare during the Malayan Emergency and World War II. Their governments subscribed to the Domino theory. Australia began by sending advisors to Vietnam in 1962, and combat troops were committed in 1965. New Zealand began by sending a detachment of engineers and an artillery battery, and then started sending special forces and regular infantry which were attached to Australian formations. Australia's peak commitment was 7,672 combat troops and New Zealand's 552. There were more than 60,000 Australian personnel involved during the course of the war, of which 521 were killed and more than 3,000 were wounded. Most served in the 1st Australian Task Force in Phước Tuy Province.
Philippines
Some 10,450 Filipino troops were dispatched to South Vietnam. They were primarily engaged in medical and other civilian pacification projects. These forces operated under the designation PHLCAG-V or Philippine Civic Action Group - Vietnam.
Thailand
Thai Army formations, including the 'Queen's Cobra' battalion, saw action in South Vietnam between 1965 and 1971. Thai forces saw much more action in the covert war in Laos between 1964 and 1972, though Thai regular formations there were heavily outnumbered by the irregular 'volunteers' of the CIA-sponsored Police Aerial Reconnaissance Units or PARU, who carried out reconnaissance activities on the western side of the Ho Chi Minh Trail.
Soviet Union
Soviet ships in the South China Sea gave vital early warnings to NLV forces in South Vietnam. The Soviet intelligence ships would pick up American B-52 bombers flying from Okinawa and Guam. Their airspeed and direction would be noted and then relayed to COSVN headquarters. COSVN using airspeed and direction would calculate the bombing target and tell any assets to move 'perpendicularly to the attack trajectory'. These advance warning gave them time to move out of the way of the bombers and while the bombing runs caused extensive damage, because of the early warnings from 1968-1970 they did not kill a single military or civilian leader in the headquarter complexes.
The Soviet Union supplied North Vietnam with medical supplies, arms, tanks, planes, helicopters, artillery, anti-aircraft missiles and other military equipment. Soviet crews fired USSR-made surface-to-air missiles at the B-52 bombers, which were the first raiders shot down over Hanoi. Fewer than a dozen Soviet citizens lost their lives in this conflict. Following the collapse of the Soviet Union in 1991, Russian officials acknowledged that the Soviet Union had stationed up to 3,000 troops in Vietnam during the war.
Some Russian sources give more specific numbers: the hardware donated by the USSR included 2,000 tanks, 7,000 artillery guns, over 5,000 anti-aircraft guns, 158 surface-to-air rocket launchers. Over the course of the war the Soviet money donated to the Vietnamese cause was equal to 2 million dollars a day. From July 1965 to the end of 1974, fighting in Vietnam was attended by some 6,500 officers and generals, as well as more than 4,500 soldiers and sergeants of the Soviet Armed Forces. In addition military schools and academies of the USSR began training Vietnamese soldiers - more than 10 thousand people.
North Korea
As a result of a decision of the Korean Workers' Party in October 1966, in early 1967 North Korea sent a fighter squadron to North Vietnam to back up the North Vietnamese 921st and 923rd fighter squadrons defending Hanoi. They stayed through 1968 and 200 pilots were reported to have served.
In addition, at least two anti-aircraft artillery regiments were sent as well. North Korea also sent weapons, ammunition and two million sets of uniforms to their comrades in North Vietnam. Kim Il-sung is reported to have told his pilots to 'fight in the war as if the Vietnamese sky were their own'.
Cuba
The extent of manpower contribution to North Vietnam by the communist Republic of Cuba, under Fidel Castro, is still a matter of debate. Then and since, the communist Vietnamese and Cuban governments have not divulged any information on this matter. There are numerous reports by former U.S. prisoners of war that Cuban military personnel were present at North Vietnamese prison facilities during the war, and that they participated in torture activities, in what is know as the 'Cuba Program'. Witnesses to this include Senator John McCain, 2008 U.S. Presidential candidate and former Vietnam prisoner of war, according to his 1999 book Faith of My Fathers. That there was at least a small contingent of Cuban military advisors present in North Vietnam during the war is without question. Some, notably Vietnam War POW/MIA issue advocates, claim evidence that Cuba's military and non-military involvement my have run into the 'thousands' of personnel.
Canada and the ICC
Canada, India and Poland comprised the International Control Commission, which was supposed to monitor the 1954 ceasefire agreement. Officially, Canada did not have partisan involvement in the Vietnam War and diplomatically it was 'non-belligerent'. Victor Levant suggested otherwise in his book, 'Quiet Complicity: Canadian Involvement in the 'Vietnam War'.
Republic of China (Taiwan)
Since November 1967, the Republic of China (Taiwan) secretly operated a cargo transport detachment to assist the United States and the ROV.
Taiwan also provided military training units for the South Vietnamese diving units, later known as the Lien Doi Nguoi Nhai (LDMN) or Frogman unit in English. In addition to the diving trainers there were several hundred military personnel. Military commandos from Taiwan were captured by communist forces three times trying to infiltrate North Vietnam