the wikipedia references to events in Phuoc Tuy Province following the Australian withdrawal are somewhat misleading. The insurgency quickly moved into the province but by 1972 the local force unit D445 had been effectively eliminated and would take a number of years to rebuild; one source states that by 1970 D445 had been reduced to the headquarters and political cadres. the regional unit was also heavily affected but by definition drew its forces from a larger area than just Phuoc Tuy.
Australian tactics were successful to the point Australian units could leave the province in significant numbers and for extended periods in order to take part in allied operations in other provinces without affecting the tactical situation in Phuoc Tuy.
Australian tactics were a development of British tactics used in Malaya, but also with a dose of US Marine procedures. the marines in IICorps essentially used similar tactics to the Australians but with less success because of the nature of the enemy effort in the area. it should be remembered that Phuoc Tuy province was not regarded as a vital area by the Viet Cong or North Viet Namese. once the initial effort to destroy the task force was comprehensively defeated at Long Tan the Australians mostly faced D445 and regional force units, neither to be compared with main force VC or regular NVA troops. the encounters with communist forces in the later stages of the deployment, when they decided, to quote one author, "to touch up" the departing Australians were against the tougher main force and NVA units where the commander of the platoon in the last serious encounter for the Australians, the aforementioned author, said that it was a close run thing between victory and massacre.
The American Army performed so poorly (possibly not the best word but i can't think of a better one) because it wasn't in Viet Nam for ten years; it was in for one year ten times. the operational and personnel experience was constantly being relearnt and a soldier returning even a year later would have been hard pressed to find significant change in the tactics or strategy of the American forces. add to this the loss of institutional experience engendered by each batch of conscripts being replaced. by the end of the war American tactics and strategy had changed, but not so much that a soldier in 1963 transported forward in time to 1973 would not have fitted in relatively easily. the same could not be said of the two world wars. ten years is a long time to be fighting without significant changes to the structure of the army, but by being forced to constantly relearn the basics it kept the institution in status quo. a more free flowing development of tactics by troops in longterm postings would have produced different results.
it is also a significant factor that at no point did the Viet Nam conflict significantly detract from American commitment to Europe and a potential conflict there. i might be mistaken, but i recall at least one source saying that more conscripts served in Europe over the course of those ten years than in Viet Nam. Westmoreland and Abrams were both highly regarded officers but it would be a bit of a stretch to compare their support to that of their counterparts in Supreme Allied Command Europe.
another factor that i don't think gets addressed in sufficient strength is the cause of the disillusionment; the constant promise of one last push to win the war. i think if the American government had left the conflict more of an open ended commitment and allowed for setbacks like Tet to upset the timetable the civilians and conscripts might have been far more patient and certainly more willing to accept political changes forced on the South Viet Namese in order to win the war. as it was any decision to change the government would only have further undermined confidence at home. the situation in Iraq and Afghanistan might be used as an indication of what it could have been like; there are hardly any protests and certainly no riots in America at the moment precisely because the military and politicians are saying that each could require a commitment of a decade or more, plus there is widespread acceptance of the need to replace Karzai in Afghanistan in order to win the war. of course the fact that these conflicts fall on a volunteer army might be a mitigating factor.