It was a process as opaque as that which took Australia into the war in the first place.

On April 22, 1970, Prime Minister John Gorton announced that one infantry battalion, 8RAR, which was due to return home in November, would not be replaced.

That was the beginning of the end of Australia's engagement in a conflict which claimed 500 Australian lives and which, though initially popular, was increasingly opposed by the community.

About a fortnight after Gorton's announcement, 120,000 people took part in the nation's first Vietnam moratorium march to protest against the war and conscription. The second, in September 1970, and the third, in June 1971, had fewer protesters but more violence and arrests.

However, government plans were driven less by growing protests than by the US, which was embarking on its own drawdown, without consulting its allies including Australia.

In June 1969, US President Richard Nixon announced the withdrawal of 25,000 American troops and in September announced the withdrawal of a further 35,000.

Increasingly disillusioned with a conflict which was costing ever more lives without apparent progress, Nixon was looking to hand the war over to South Vietnamese forces in a face-saving process termed Vietnamisation, then get out.

Nixon's national security adviser Henry Kissinger cynically and secretly sought to achieve a "decent interval" before the seemingly inevitable victory of the communist North.

Australian soldiers had first gone to Vietnam in 1962, with 30 specialist military advisers despatched at the request of the US to assist South Vietnamese forces counter the growing communist insurgency.

The big decision came in April 1965, with the deployment of an Army combat battalion, 1RAR, which arrived in May and June. More followed in 1966, creating a task force which in May 1969 peaked at more than 5000.

This was a big commitment for Australia, but was utterly dwarfed by the US whose troop numbers peaked at 543,000 at the same time.

Ashley Ekins, former Australian War Memorial principal historian and author of the final volumes of the official history of Australia's war in Vietnam "Fighting to the Finish", said Washington's withdrawal announcements threw the Australian government into disarray.

"We were a very minor player, although this was a major commitment to us. It became a heavy burden and cost," Ekins said.

"The government kept trying to claim our withdrawal intentions were linked to those of the US as part of a larger plan."

When the Australian Defence Department began planning for a withdrawal, Chief of the General Staff Lieutenant General Tom Daly and Chairman of the Chiefs of Staff Committee, General John Wilton, concluded it should be "one out all out".

They argued that piecemeal withdrawal would make the task force, based at Nui Dat and responsible for security of the sprawling Phuoc Tuy province, less effective and more vulnerable to enemy action.

Senior foreign affairs officials took the view that a phased withdrawal would better keep faith with the Americans.

That view, backed by new Defence Minister Malcolm Fraser, prevailed and on April 22, 1970 Gorton, who initially favoured the "one out all out" approach, stood up in parliament.

He announced that 8RAR plus some support personnel would return at the end of their tour in November and not be replaced. Further withdrawals would be considered as Vietnamisation proceeded.

Labor Opposition Leader Gough Whitlam dismissed any pretence that this was an independent decision based on the military situation in Phuoc Tuy, saying it was clearly made after the US announcement.

Shortly after 8RAR returned home, New Zealand also withdrew a rifle company, further reducing the task force capability.

In four years, Australian and New Zealand troops had achieved considerable success in Phuoc Tuy.

At its peak, the task forces comprised three infantry battalions plus supporting units including tanks, armoured personnel carriers, helicopters and engineers.

That enabled one battalion to be constantly rotated through field operations, maintaining pressure on insurgent forces while the other two conducted local operations and guarded the base.

One less battalion meant the troops had to work harder for less effect.

Ekins said Gorton's decision was made against the advice of commanders and soldiers on the ground.

"Duty bound as they were constitutionally, the service chiefs had to abide," he said. "The Army could make those required adjustments but they knew it would be at a cost."

The protracted withdrawal took two years. Only in March 1972 did the final task force soldiers come home.

"In that time another 100 had died and more than 1000 had been wounded," Ekins said.

The last of the Centurion tanks departed Vietnam in September 1971 - also against the commanders' advice that they should stay until the end to provide the infantry with the mobile firepower they needed against enemy bunkers.

Just how the risk had increased was demonstrated in the last major Australian action of the conflict, Operation Ivanhoe in September 1971. Soldiers of 3RAR and 4RAR/NZ attacked enemy positions without armour support, resulting in five killed and 30 wounded.

On August 18, 1971 - the fifth anniversary of the Battle of Long Tan - Prime Minister Billy McMahon, who succeeded Gorton in March 1971, announced the two remaining battalions would be home by Christmas.

Australian combat operations effectively ended with their withdrawal, although the war would continue for another four years until the crushing victory by North Vietnamese forces in April 1975.

Australian Associated Press