With regards to which part? Is this question referring to the US involvement which ends in 1973 or the conclusion and fall of Saigon in 1975?

The basic colliding forces were the determination of the USA, with great resources and confidence and at the height of its power, and the determination of the regime of North Vietnam, which had defeated the French and regarded itself as the legitimate standard-bearer of Vietnamese independence and unification.

Each side was able to match the escalation of the other. The US was limited ultimately by the limited objectives of the war and it's greater priority in Europe and to counter the USSR and China generally. It was hamstrung by the corrupt nature of the regimes it sponsored in the South, the fact that its military was basically set up for a European style war against an industrial power, and the fact that the war was never popularised nor seen as vital for US interests specifically, though it did have enormous symbolic value and thus defeat in Vietnam would have been a massive blow to prestige at a time when the ideologies of communism and capitalism were in a stern confrontation- with many proxy wars being fought all over the world. These factors exacerbated the issues of using a conscript army.

The socialist/nationalist Vietnamese were limited in military capability and resources, but had the advantage of a comprehensive apparatus to administer areas once occupied, and an ability to scale operations up or down to match resources. They also saw themselves as fighting for independence, taking part in a war of enormous national significance.

By the early 1970s the US was tired of the war: financial resources came under pressure, the public were less enthusiastaic, and the war conflated with social issues at home like drugs and class- and race-conflict. 'Victory' was no longer the objective, but 'peace with honour'. The Southern regime the US had built up had become reasonably competent and legitimate, its military quite strong (the South Vietnamese Army [ARVN] always fought on a larger scale than the lesser numbers of US- and allied troops, and suffered more casualties). The efforts of several large scale, unsuccessful offensives (Tet, the 1972 'Easter Offensive') had sapped the forces of North Vietnam, too. Both sides thus had a little more motivation to find a peace treaty, albeit one seen only as a temporary suspension by the North. The South had limited input into these talks.

By 1973 the US had withdrawn most of her forces, having relegated them to defensive operations and building up the ARVN. While the North consolidated and prepared, domestic US policymakers starved the Southern government of aid money and over time their US-modelled military was run down, losing much of its high-tech and heavy weapons capability.

In 1974-75 fresh fighting broke out, centred on the Central Highlands through which the North could supply its forces with much less interdiction. The Northern forces were able to out-fight those of the South, taking control of the country in April 1975 while the US refused to become involved.