Conventional wisdom held and still holds that the war was 'unwinnable' and it was the height of American hubris to believe our satrapy could prevail against the pure forces of Vietnamese nationalism and desire for sovereignty.
Vietnam war scholar Stephen Morris argues that:
This simple explanation is repudiated by powerful historical evidence, both old and new. Its proponents mistakenly base their conclusions on the situation in Vietnam during the 1950's and early 1960's and ignore the changing course of the war (notably, the increasing success of President Richard Nixon's Vietnamization strategy) and the evolution of South Vietnamese society (in particular the introduction of agrarian reforms).
For all the claims of popular support for the Vietcong insurgency, far more South Vietnamese peasants fought on the side of Saigon than on the side of Hanoi. The Vietcong were basically defeated by the beginning of 1972, which is why the North Vietnamese launched a huge conventional offensive at the end of March that year. During the Easter Offensive of 1972 - at the time the biggest campaign of the war - the South Vietnamese Army was able to hold onto every one of the 44 provincial capitals except Quang Tri, which it regained a few months later. The South Vietnamese relied on American air support during that offensive.
If the United States had provided that level of support in 1975, when South Vietnam collapsed in the face of another North Vietnamese offensive, the outcome might have been at least the same as in 1972. But intense lobbying of Congress by the antiwar movement, especially in the context of the Watergate scandal, helped to drive cutbacks of American aid in 1974. Combined with the impact of the world oil crisis and inflation of 1973-74, the results were devastating for the south. As the triumphant North Vietnamese commander, Gen. Van Tien Dung, wrote later, President Nguyen Van Thieu of South Vietnam was forced to fight "a poor man's war."
Even Hanoi's main patron, the Soviet Union, was convinced that a North Vietnamese military victory was highly unlikely. Evidence from Soviet Communist Party archives suggests that, until 1974, Soviet military intelligence analysts and diplomats never believed that the North Vietnamese would be victorious on the battlefield. Only political and diplomatic efforts could succeed. Moscow thought that the South Vietnamese government was strong enough to defend itself with a continuation of American logistical support. The former Soviet chargé d'affaires in Hanoi during the 1970's told me in Moscow in late 1993 that if one looked at the balance of forces, one could not predict that the South would be defeated. Until 1975, Moscow was not only impressed by American military power and political will, it also clearly had no desire to go to war with the United States over Vietnam. But after 1975, Soviet fear of the United States dissipated.
What is also clear from Soviet archival sources is that those who believed that North Vietnam had more than national unification on its mind were right: Its leaders were imbued with a sense of their ideological mission - not only to unify Vietnam under Communist Party rule, but also to support the victory of Communists in other nations. They saw themselves as the outpost of world revolution in Southeast Asia and desired to help Communists in Cambodia, Laos, Thailand and elsewhere.