The Vietnam War involved many countries across the world. North Vietnam received support from the Eastern Bloc, while South Vietnam was supported by nations of the Western Bloc.

Pro-North Vietnam

China

In 1950, the People's Republic of China extended diplomatic recognition to the Democratic Republic of Vietnam led by the communist Viet Minh and sent heavy weapons, as well as military advisers 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, seeing U.S. intervention coming, urged the Viet Minh to accept a partition at the 17th parallel.

According to US diplomat, Richard Holbrooke, ongoing Sino-Soviet tensions Chinese and Russian mutual suspicions were suspended when they united against US forces in Vietnam.

China's support for North Vietnam included both financial aid initially, and later included the deployment of hundreds of thousands of military personnel in support roles. 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, man anti-aircraft batteries, rebuild roads and railroads, transport supplies, and perform other engineering works. Chinese logistic support freed North Vietnamese army units for combat in the South. China sent 320,000 troops and annual arms shipments worth $180 million. The Chinese military claims to have caused 38% of American air losses in the war.

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 had refused. The Chinese began to withdraw some units in November 1968 in preparation for a clash with the Soviets, which occurred at Zhenbao Island in March 1969.

In 1967, the Chinese government launched a secret military program named "Project 523". which sought to develop an antimalarial medication due to widespread malaria among PAVN forces and Chinese scientist Tu Youyou and her collaborators discovered artemisinin. Tu was awarded the Nobel Prize in 2015 for her contribution on the anti-malaria treatment.

The PRC also began financing the Khmer Rouge as a counterweight to North Vietnam during the Cambodian Civil War. China "armed and trained" the Khmer Rouge during the civil war and continued to aid them for years afterward. The Khmer Rouge launched raids into Vietnam in 1975–1978. When Vietnam responded with an invasion that toppled the Khmer Rouge, China launched a brief, punitive invasion of Vietnam in 1979.

Soviet Union

The Soviet Union provided military assistance by assisting and training North Vietnamese air force personnel. Soviet pilots sometimes flew the planes their government supplied.

Soviet ships in the South China Sea gave early warning intel to PAVN/VC 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 recorded and relayed to the Central Office for South Vietnam, which was the PAVN/VC's southern headquarters. Using airspeed and direction, COSVN analysts calculated bombing targets and directed assets to move "perpendicularly to the attack trajectory." These advance warnings gave military units 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 to 1970 they did not kill a single military or civilian leader in the headquarters complexes.

The Soviet Union also helped North Vietnam develop anti-US propaganda posters.

The Soviet Union supplied North Vietnam with medical supplies, arms, tanks, planes, helicopters, artillery, anti-aircraft missiles and other military equipment. Soviet crews fired Soviet-made surface-to-air missiles at U.S. F-4 Phantoms, which were shot down over Thanh Hóa in 1965. Over a dozen Soviet soldiers lost their lives in this conflict. Following the dissolution of the Soviet Union in 1991, Russian Federation 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: Between 1953 and 1991, the hardware donated by the Soviet Union included 2,000 tanks, 1,700 APCs, 7,000 artillery guns, over 5,000 anti-aircraft guns, 158 surface-to-air missile launchers, and 120 helicopters. During the war, the Soviets sent North Vietnam annual arms shipments worth $450 million. From July 1965 to the end of 1974, fighting in Vietnam was observed by some 6,500 officers and generals, as well as more than 4,500 soldiers and sergeants of the Soviet Armed Forces. In addition, Soviet military schools and academies began training Vietnamese soldiers—in all more than 10,000 military personnel.

The KGB also helped to develop the signals intelligence (SIGINT) capabilities of North Vietnam, through an operation known as Vostok (also known as Phương Đông, meaning "Orient" and named after the Vostok 1). The Vostok program was a counterintelligence and espionage program. These programs were pivotal in detecting and defeating CIA and South Vietnamese commando teams sent into North Vietnam, as they were detected and captured. The Soviets helped the Ministry of Public Security recruit foreigners within high-level diplomatic circles among the Western-allies of the US, under a clandestine program known as "B12,MM" which produced thousands of high-level documents for nearly a decade, including targets of B-52 strikes. In 1975, the SIGINT services had broken information from Western US-allies in Saigon, determining that the US would not intervene to save South Vietnam from collapse.

Other communist states

Czechoslovakia The Czechoslovak Socialist Republic was a member of the Warsaw Pact and sent significant aid to North Vietnam, both prior to and after the Prague Spring. The Czechoslovak government created committees which sought to promote and establish peace, while also promoting victory for Viet Cong and PAVN forces. Czech-made equipment and military aid increased significantly following the Prague Spring. Czechoslovakia continued to send tens of thousands of Czech-made rifles as well as mortar and artillery throughout the war. In general, Czechoslovakia was aligned with European leftist movements, and there were simultaneous protests demonstrating against the Soviet intervention in Prague and the US intervention in Vietnam.

Cooperation with Czechoslovakia on the development of North Vietnamese air capabilities began as early as 1956. Czechoslovak instructors and trainers instructed the Vietnam People's Air Force (VPAF) in China and helped them develop a modernised air force, with the Czech-built Aero Ae-45 and Aero L-29 Delfín alongside Zlín Z 26 aircraft utilised significantly for training, and regarded as preferential to Soviet-built Yakovlev Yak-3 as training aircraft.

North Korea The Korean Workers' Party decided in October 1966, in early 1967, that North Korea would send a Korean People's Army Air Force fighter squadron to North Vietnam to back up the North Vietnamese 921st and 923rd Fighter Squadrons defending Hanoi. The North Koreans stayed through 1968, and 200 pilots were reported to have served. In addition, at least two anti-aircraft artillery regiments were sent as well.

Cuba

The contributions to North Vietnam's war effort by the Republic of Cuba under Fidel Castro have been recognized several times by representatives of the Democratic Republic of Vietnam. Castro mentioned in his discourses the Batallón Girón (Giron Battalion) as comprising the Cuban contingent that served as military advisors during the war. In this battalion, the Cubans were aided by Nguyễn Thị Định, founding member of the Viet Cong, who later became the first female major general in the PAVN.

There are numerous allegations by former U.S. prisoners of war that Cuban Revolutionary Armed Forces personnel were present at North Vietnamese prison facilities during the war and that they participated in torture activities. Witnesses to this include Senator John McCain, the 2008 U.S. presidential candidate and a former Vietnam prisoner of war, according to his 1999 book Faith of My Fathers.

East Germany The Ministry of Public Security of Vietnam (Bộ Công An) states that there was special interest towards the Stasi of East Germany in establishing an intelligence and security apparatus, particularly since the Stasi was well-regarded due to their industrial, modern, and (with a) scientific working-style. In official Vietnamese language histories on the Vietnamese Ministry of Public Security, the assistance provided by the Soviet and East German intelligence services to Vietnam is usually rated as among the most important within the socialist bloc. East Germany had also assisted North Vietnam in duplicating "Green Dragon" identity cards used in South Vietnam, which were created by Saigon in order to identify North Vietnamese combatants from citizens, and were generally difficult to duplicate.

East German authorities had also begun providing material and technical aid to help develop and modernise the North Vietnamese economy and military. In addition, East Germany had also vigorously denounced the US war effort, and had reaped significant international and diplomatic standing as a result of its anti-war campaigns.

Romania Romania was also among primary supporters of North Vietnam during the war in political, economic and military terms. Contemporarily, the Eastern Bloc country was also known for its role in the mediation activities in the mid-1960s, resulting in what became known as the "Trinh Signal" in January 1967, in which Hanoi accepted the possibility of negotiation with Washington.

Bulgaria Bulgaria committed their charge-free military and economic supplies to North Vietnam in a bilateral agreement signed in 1972. Bulgarian military aid had already been provided to the latter since 1967. Similar conducts was undertaken by Hungary, which was reaffirmed in mutual visits of Hungary and North Vietnam in 1972 and 1973.

Hungary Hungary also expressed their support through their representatives at the International Commission of Control and Supervision, a body established to supervise the implementation of the Paris Peace Accords.

Pro-South Vietnam

As South Vietnam was formally part of a military alliance with the US, Australia, New Zealand, France, the UK, Pakistan, Thailand and the Philippines, the alliance was invoked during the war. The UK, France and Pakistan declined to participate, and South Korea and Taiwan were non-treaty participants.

United States

The United States was the main ally of France and the State of Vietnam associated with the French Union in the First Indochina War against the communist-led Viet Minh. It recognized the State of Vietnam on 7 February 1950. After Vietnam was divided in 1954 and the State of Vietnam became the Republic of Vietnam (known as "South Vietnam"), the United States was Saigon's main and direct ally in the Vietnam War against the communist North.

Other anti-communist states

South Korea