The Bush Administration has always refused to make any comparisons between Iraq and the Vietnam War, the conflict that traumatised a generation of Americans. Washington does not want public opinion to associate the struggle in Iraq with the 20-year conflict in Vietnam, which cost America almost 58,000 dead and about 150,000 seriously wounded.

Today, however, for the first time President Bush explicitly drew the comparison when he warned Americans that a precipitate withdrawal from Vietnam could trigger upheavals potentially as disastrous as those that followed America’s final withdrawal from Saigon in 1975.

How valid are any comparisons, and how misleading is it to look back 30 years either to justify the present engagement or to argue for a swift pull-out?

The War. American engagement in South-East Asia began in 1961, when President Kennedy ordered new equipment, 3,000 military advisers and US helicopters to be sent to help South Vietnam’s government in its war against Vietcong guerrillas. It ended, memorably, on April 30, 1975, when the last Marines guarding the US embassy in Saigon lifted off from the roof.

By contrast, there was no ongoing conflict in Iraq before the coalition invasion in March, 2003. At their maximum strength during the invasion, US troops peaked at 175,000. With the current surge they number 160,000.

Outside intervention. South Vietnam was fighting against a regular army of troops backed by North Vietnam and armed by Russia and China. In the Tet offensive alone, 37,000 were killed. In Iraq, al-Qaeda and terrorists from other countries number between 3,000 - 5,000. The insurgency receives military and logistical backing largely from Iran. No other country has sent arms or personnel to fight the Americans.

Negotiations. The US spent two years negotiating directly with North Vietnam to end the war, finally signed a ceasefire in January 1973. In Iraq, no formal negotiations have taken place with any insurgent group, and only two brief sets of talks have been held with low-level Iranian officials.

Withdrawal. In Vietnam, US forces were withdrawn over a two-year period, with only 133,000 remaining by January 1972 — a third of the earlier strength. The ground fighting became the responsibility of South Vietnam, which had more than 1,000,000 men enlisted. In Iraq, no set plans have been made for withdrawal — and the Iraqi Army is about a tenth of the size of South Vietnam’s.

Consequences. After pull-out from Vietnam, thousands of former US allies, government workers, intellectuals and businessmen were sent to prison camps and hundreds of thousands more fled abroad in boats, as President Bush noted today. In neighbouring Cambodia, the Khmer Rouge succeeded in ousting the pro-American government and began a three-year murderous rule.

In Iraq, fear of civil war between Shias and Sunnis is a main threat if all foreign forces were withdrawn now, with possible open intervention by both Iran, in support of the Shia, and Saudi Arabia in support of the minority Sunnis. There is little insurgency threat to the governments of Turkey, Iran and Syria, but the governments of the Gulf states and Saudi Arabia are deeply concerned that al-Qaeda and Islamists might redouble efforts to overthrow them if the US pulled out of the entire region.

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