With the northern half of the country under their control, the Politburo ordered General Dung to launch the final offensive against Saigon. The operational plan for the Ho Chi Minh Campaign called for the capture of Saigon before 1 May. Hanoi wished to avoid the coming monsoon and prevent any redeployment of ARVN forces defending the capital. Northern forces, their morale boosted by their recent victories, rolled on, taking Nha Trang, Cam Ranh, and Da Lat.

On 7 April, three North Vietnamese divisions attacked Xuan Loc, 40 miles (64 km) east of Saigon. The North Vietnamese met fierce resistance at Xuan Loc from the ARVN 18th Division, who were outnumbered six to one. For two bloody weeks, severe fighting raged as the ARVN defenders made a last stand to try to block the North Vietnamese advance. By 21 April, however, the exhausted garrison were ordered to withdraw towards Saigon.

An embittered and tearful President Thieu resigned on the same day, declaring that the United States had betrayed South Vietnam. In a scathing attack, he suggested U.S. Secretary of State Henry Kissinger had tricked him into signing the Paris peace agreement two years ago, promising military aid that failed to materialise.

At the time of the peace agreement the United States agreed to replace equipment on a one-by-one basis.But the United States did not keep its word. Is an American's word reliable these days? The United States did not keep its promise to help us fight for freedom and it was in the same fight that the United States lost 50,000 of its young men.

Having transferred power to Tran Van Huong, he left for Taiwan on 25 April. At the same time, North Vietnamese tanks had reached Bien Hoa and turned toward Saigon, brushing aside isolated ARVN units along the way.

By the end of April, the Army of the Republic of South Vietnam had collapsed on all fronts. Thousand of refugees streamed southward, ahead of the main communist onslaught. On 27 April 100,000 North Vietnamese troops encircled Saigon. The city was defended by about 30,000 ARVN troops. To hasten a collapse and foment panic, the VPA shelled the airport and forced its closure. With the air exit closed, large numbers of civilians found that they had no way out.

On 10 March 1975, General Dung launched Campaign 275, a limited offensive into the Central Highlands, supported by tanks and heavy artillery. The target was Ban Me Thuot, in Daklak Province. If the town could be taken, the provincial capital of Pleiku and the road to the coast would be exposed for a planned campaign in 1976. The ARVN proved incapable of resisting the onslaught, and its forces collapsed on 11 March. Once again, Hanoi was surprised by the speed of their success. Dung now urged the Politburo to allow him to seize Pleiku immediately and then turn his attention to Kon Tum. He argued that with two months of good weather remaining until the onset of the monsoon, it would be irresponsible to not take advantage of the situation.

President Nguyễn Văn Thiệu, a former general, was fearful that his forces would be cut off in the north by the attacking communists; Thieu ordered a retreat. The president declared this to be a "lighten the top and keep the bottom" strategy. But in what appeared to be a repeat of Operation Lam Son 719, the withdrawal soon turned into a bloody rout. While the bulk of ARVN forces attempted to flee, isolated units fought desperately. ARVN General Phu abandoned Pleiku and Kon Tum and retreated toward the coast, in what became known as the "column of tears".

As the ARVN tried to disengage from the enemy, refugees mixed in with the line of retreat. The poor condition of roads and bridges, damaged by years of conflict and neglect, slowed Phu's column. As the North Vietnamese forces approached, panic set in. Often abandoned by the officers, the soldiers and civilians were shelled incessantly. The retreat degenerated into a desperate scramble for the coast. By 1 April the "column of tears" was all but annihilated.

On 20 March, Thieu reversed himself and ordered Huế, Vietnam's third-largest city, be held at all costs, and then changed his policy several times. Thieu's contradictory orders confused and demoralized his officer corps. As the North Vietnamese launched their attack, panic set in, and ARVN resistance withered. On 22 March, the VPA opened the siege of Huế. Civilians flooded the airport and the docks hoping for any mode of escape. Some even swam out to sea to reach boats and barges anchored offshore. In the confusion, routed ARVN soldiers fired on civilians to make way for their retreat.

On 31 March, after a three-day battle, Huế fell. As resistance in Huế collapsed, North Vietnamese rockets rained down on Da Nang and its airport. By 28 March, 35,000 VPA troops were poised to attack the suburbs. By 30 March 100,000 leaderless ARVN troops surrendered as the VPA marched victoriously through Da Nang. With the fall of the city, the defense of the Central Highlands and Northern provinces came to an end.

The Vietnam War summary aims to enable readers to find out the brief history of Vietnam war. It may be quite helpful.

The summary of Vietnam War:

Date
1 November 1955 – 15 May 1975 (1975-05-15) (&000000000000001900000019 years, &0000000000000195000000195 days)

Belligerents
Anti Communist: United States, South Vietnam, South Korea, Australia, Philippines, New Zealand, Thailand, Khmer Republic, Republic of Laos, Republic of China (Taiwan), Germany

Communist: North Vietnam, Vietcong, Soviet Union, North Korea, People's Republic of China, Khmer Rogue, Pathet Lao.

Military Forces
Anti Communist:

• South Vietnam: 850,000
• United States: 536,100
• Free World Forces: 65,000
• South Korea: 312,853,
• Australia: 49,968 (1962–1973)
• Thailand, Philippines: 10,450
• New Zealand: 3,890 (1964–1973)

Communist:

• North Vietnam: 340,000
• PRC: 170,000 (1969)
• Soviet Union: 3,000
• North Korea: 300

Causes
The causes of the Vietnam War trace their roots back to the end of World War II. A French colony, Indochina (Vietnam, Laos, & Cambodia) had been occupied by the Japanese during the war.

In 1941, a Vietnamese nationalist movement, the Viet Minh, was formed by Ho Chi Minh to resist the occupiers. A communist, Ho Chi Minh waged a guerilla war against the Japanese with the support of the United States. Near the end of the war, the Japanese began to promote Vietnamese nationalism and ultimately granted the country nominal independence.

Following the Japanese defeat, the French returned to take possession of their colony. Their entrance into Vietnam was only permitted by the Viet Minh after assurances had been given that the country would gain independence as part of the French Union. Discussions broke down between the two parties and in December 1946, the French shelled the city of Haiphong and forcibly reentered the capital, Hanoi.

These actions began a conflict between the French and the Viet Minh known as the First Indochina War. The war was ultimately settled by the Geneva Accords of 1954, which temporarily partitioned the country at the 17th parallel, with the Viet Minh in control of the north and a non-communist state to be formed in the south under Prime Minister Ngo Dinh Diem. This division was to last until 1956, when national elections would be held to decide the future of the nation.

The United States had little interest in Vietnam and Southeast Asia, however as it became clear that the post-World War II world would be dominated by the US and its allies and the Soviet Union and theirs, isolating communist movements took an increased importance.

In 1950, to combat the spread of Communism, the United States began supplying the French military in Vietnam with advisors and funding its efforts against the “red” Viet Minh. These efforts continued in 1956, when advisors were provided to train the army of the new Republic of Vietnam (South Vietnam). Despite their best efforts, the quality of the Army of the Republic of Vietnam (ARVN) was to remain consistently poor throughout its existence.

Americanization
On 2 August 1964, the USS Maddox, on an intelligence mission along North Vietnam's coast, allegedly fired upon and damaged several torpedo boats that had been stalking it in the Gulf of Tonkin. Following this attack, Congress passed the Southeast Asia Resolution which allowed President Lyndon Johnson to conduct military operations in the region without a declaration of war. On March 2, 1965, US aircraft began bombing targets in Vietnam and the first troops arrived. Commanded by General William Westmoreland, US troops won victories over Viet Cong and North Vietnamese forces around Chu Lai and in the Ia Drang Valley that summer.

Ground War
On 8 March 1965, 3,500 United States Marines were dispatched to South Vietnam. This marked the beginning of the American ground war. U.S. public opinion overwhelmingly supported the deployment. The Marines' assignment was defensive. The initial deployment of 3,500 in March was increased to nearly 200,000 by December.

Test Offensive
The Tet Offensive was a military campaign during the Vietnam War that began on January 31, 1968. Forces of the National Liberation Front for South Vietnam (NLF, or Viet Cong), and the People's Army of Vietnam (the North Vietnamese army), fought against the forces of the Republic of Vietnam (South Vietnam), the United States, and their allies. The purpose of the offensive was to strike military and civilian command and control centers throughout South Vietnam and to spark a general uprising among the population that would then topple the Saigon government, thus ending the war in a single blow.

Vietnamization
Severe communist losses during the Tet Offensive allowed U.S. President Richard M. Nixon to begin troop withdrawals. His plan, called the Nixon Doctrine, was to build up the ARVN, so that they could take over the defense of South Vietnam. The policy became known as "Vietnamization". Vietnamization had much in common with the policies of the Kennedy administration. One important difference, however, remained. While Kennedy insisted that the South Vietnamese fight the war themselves, he attempted to limit the scope of the conflict.

End of The War
The United States began drastically reducing their troop support in South Vietnam during the final years of "Vietnamization". Many U.S. troops were removed from the region, and on 5 March 1971, the United States returned the 5th Special Forces Group, which was the first American unit deployed to South Vietnam, to its former base in Fort Bragg, North Carolina. By March of that year, American combat troops had left the country. After a brief period of peace, North Vietnam recommenced hostilities in late 1974. Pushing through ARVN forces with ease, they captured the Saigon on April 30, 1975, forcing South Vietnam’s surrender and reuniting the country. On 10 March 1975, General Dung launched Campaign 275, a limited offensive into the Central Highlands, supported by tanks and heavy artillery. The target was Ban Me Thuot, in Daklak Province. If the town could be taken, the provincial capital of Pleiku and the road to the coast would be exposed for a planned campaign in 1976.

Aftermath
Phnom Penh, the capital of Cambodia, fell to followers of the Communist Party of Kampuchea, commonly known as the Khmer Rouge, on 17 April 1975. Over the next four years, the Khmer Rouge enacted a genocidal policy that killed over one-fifth of all Cambodians, or more than a million people. After repeated border clashes in 1978, Vietnam invaded Democratic Kampuchea (Cambodia) and ousted the Khmer Rouge in the Cambodian–Vietnamese War.

In the post-war era, Americans struggled to absorb the lessons of the military intervention. As General Maxwell Taylor, one of the principal architects of the war, noted "first, we didn't know ourselves. We thought that we were going into another Korean War, but this was a different country. Secondly, we didn't know our South Vietnamese allies... And we knew less about North Vietnam. Who was Ho Chi Minh? Nobody really knew. So, until we know the enemy and know our allies and know ourselves, we'd better keep out of this kind of dirty business. It's very dangerous."

Casualties
United States: 58,119 killed, 153,303 wounded, 1,948 missing in action
South Vietnam: 230,000 killed and 1,169,763 wounded (estimated)
North Vietnam: 1,100,000 killed in action (estimated) and an unknown number of wounded

The United States began drastically reducing their troop support in South Vietnam during the final years of "Vietnamization". Many U.S. troops were removed from the region, and on March 5, 1971, the United States returned the 5th Special Forces Group, which was the first American unit deployed to South Vietnam, to its former base in Fort Bragg, North Carolina.

Under the Paris Peace Accord, between North Vietnamese Foreign Minister Lê Ðức Thọ and U.S. Secretary of State Henry Kissinger, and reluctantly signed by South Vietnamese President Thiệu, U.S. military forces withdrew from South Vietnam and prisoners were exchanged. North Vietnam was allowed to continue supplying communist troops in the South, but only to the extent of replacing materials that were consumed. Later that year the Nobel Peace Prize was awarded to Kissinger and Thọ, but the Vietnamese negotiator declined it saying that a true peace did not yet exist.

The communist leaders had expected that the ceasefire terms would favor their side. But Saigon, bolstered by a surge of U.S. aid received just before the ceasefire went into effect, began to roll back the Vietcong. The communists responded with a new strategy hammered out in a series of meetings in Hanoi in March 1973, according to the memoirs of Trần Văn Trà.

As the Vietcong's top commander, Trà participated in several of these meetings. With U.S. bombings suspended, work on the Ho Chi Minh Trail and other logistical structures could proceed unimpeded. Logistics would be upgraded until the North was in a position to launch a massive invasion of the South, projected for the 1975–1976 dry season. Trà calculated that this date would be Hanoi's last opportunity to strike before Saigon's army could be fully trained.

Although McGovern himself was not elected U.S. president, the November 1972 election did return a Democratic majority to both houses of Congress under McGovern's "Come home America" campaign theme. On March 15, 1973, U.S. President Richard Nixon implied that the United States would intervene militarily if the communist side violated the ceasefire. Public and congressional reaction to Nixon's trial balloon was unfavorable and in April Nixon appointed Graham Martin as U.S. ambassador to Vietnam. Martin was a second stringer compared to previous U.S. ambassadors and his appointment was an early signal that Washington had given up on Vietnam. During his confirmation hearings in June 1973, Secretary of Defense James R. Schlesinger stated that he would recommend resumption of U.S. bombing in North Vietnam if North Vietnam launched a major offensive against South Vietnam. On June 4, 1973, the U.S. Senate passed the Case-Church Amendment to prohibit such intervention.

The oil price shock of October 1973 caused significant damage to the South Vietnamese economy. The Vietcong resumed offensive operations when dry season began and by January 1974 it had recaptured the territory it lost during the previous dry season. After two clashes that left 55 South Vietnamese soldiers dead, President Thiệu announced on January 4 that the war had restarted and that the Paris Peace Accord was no longer in effect. There had been over 25,000 South Vietnamese casualties during the ceasefire period.

Gerald Ford took over as U.S. president on August 9, 1974 after President Nixon resigned due to the Watergate scandal. At this time, Congress cut financial aid to South Vietnam from $1 billion a year to $700 million. The U.S. midterm elections in 1974 brought in a new Congress dominated by Democrats who were even more determined to confront the president on the war. Congress immediately voted in restrictions on funding and military activities to be phased in through 1975 and to culminate in a total cutoff of funding in 1976.

The success of the 1973–1974 dry season offensive inspired Trà to return to Hanoi in October 1974 and plead for a larger offensive in the next dry season. This time, Trà could travel on a drivable highway with regular fueling stops, a vast change from the days when the Ho Chi Minh Trail was a dangerous mountain trek. Giáp, the North Vietnamese defense minister, was reluctant to approve Trà's plan. A larger offensive might provoke a U.S. reaction and interfere with the big push planned for 1976. Trà appealed over Giáp's head to first secretary Lê Duẩn, who approved of the operation.

Trà's plan called for a limited offensive from Cambodia into Phuoc Long Province. The strike was designed to solve local logistical problems, gauge the reaction of South Vietnamese forces, and determine whether U.S. would return to the fray.

On December 13, 1974, North Vietnamese forces attacked Route 14 in Phuoc Long Province. Phuoc Binh, the provincial capital, fell on January 6, 1975. Ford desperately asked Congress for funds to assist and re-supply the South before it was overrun. Congress refused. The fall of Phuoc Binh and the lack of an American response left the South Vietnamese elite demoralized.

The speed of this success led the Politburo to reassess its strategy. It was decided that operations in the Central Highlands would be turned over to General Văn Tiến Dũng and that Pleiku should be seized, if possible. Before he left for the South, Dũng was addressed by Lê Duẩn: "Never have we had military and political conditions so perfect or a strategic advantage as great as we have now."

At the start of 1975, the South Vietnamese had three times as much artillery and twice the number of tanks and armoured cars as the opposition. They also had 1,400 aircraft and a two-to-one numerical superiority in combat troops over their Communist enemies. However, the rising oil prices meant that much of this could not be used. They faced a well-organized, highly determined and well-funded North Vietnam. Much of the North's material and financial support came from the communist bloc. Within South Vietnam, there was increasing chaos. Their abandonment by the American military had compromised an economy dependent on U.S. financial support and the presence of a large number of U.S. troops. South Vietnam suffered from the global recession that followed the Arab oil embargo.

Some advocates within the peace movement advocated a unilateral withdrawal of U.S. forces from Vietnam. One reason given for the withdrawal is that it would contribute to a lessening of tensions in the region and thus less human bloodshed. Early opposition to U.S. involvement in Vietnam was centered around the Geneva conference of 1954. American support of Diem in refusing elections was thought to be thwarting the very democracy that America claimed to be supporting. John Kennedy, while Senator, opposed involvement in Vietnam.

Opposition to the Vietnam War tended to unite groups opposed to U.S. anti-communism, imperialism and colonialism and, for those involved with the New Left such as the Catholic Worker Movement, capitalism itself. Others, such as Stephen Spiro opposed the war based on the theory of Just War. Some wanted to show solidarity with the people of Vietnam, such as Norman Morrison emulating the actions of Thích Quảng Đức. Some critics of U.S. withdrawal predicted that it would not contribute to peace but rather vastly increase bloodshed. These critics advocated U.S. forces remain until all threats from the Viet Cong and North Vietnamese Army had been eliminated. Advocates of U.S. withdrawal were generally known as "doves", and they called their opponents "hawks", following nomenclature dating back to the War of 1812.

High-profile opposition to the Vietnam War turned to street protests in an effort to turn U.S. political opinion. On October 15, 1969, the Vietnam Moratorium attracted millions of Americans.

The fatal shooting of four anti-war protesters at Kent State University led to nation-wide university protests. Riots broke out at the 1968 Democratic National Convention.

After explosive news reports of American military abuses, such as the 1968 My Lai Massacre, brought new attention and support to the anti-war movement, some veterans joined Vietnam Veterans Against the War.

Anti-war protests ended with the final withdrawal of troops after the Paris Peace Accords were signed in 1973. South Vietnam was left to defend itself alone when the fighting resumed. Many South Vietnamese fled to the United States in one of the largest war refugee migrations in history.