From the moment that US forces invaded Iraq, President Bush’s critics have invoked the spectre of Vietnam as the clinching argument against the war, the unanswerable case for early disengagement from a deepening debacle. From that same moment, Mr Bush and his supporters have resisted forcefully the idea that there was any parallel.

The military capabilities of the US were much greater in the early 21st century than in the 1970s, they said. The Iraqi people would be much more easily pacified than were the North Vietnamese. Above all, the political conditions in the two regions were sharply different.

But yesterday, in a striking inversion of the terms of that debate, Mr Bush insisted that Vietnam was indeed the model. Not as a lesson in how not to get bogged down in a messy war thousands of miles away; but that failure to finish the job in SouthEast Asia had created a calamity for the region and the world.

Mr Bush said that the state of modern Asia — dynamic and, in political terms, much freer than it was 60 years ago — owed much to the US engagement there in the Second World War and afterwards.

But he saved his most passionate words to criticise American policy. The decision to leave Vietnam in 1975 left the region prey to an eruption of violence and tyranny. In Vietnam itself, millions suffered under communist rule. In Cambodia, Pol Pot’s murderous regime ran riot over its domestic and foreign enemies. Elsewhere — in Laos and even in China — the mess caused by the US-backed war and its departure before it had stabilised Vietnam caused misery and chaos.

Above all, the knowledge that the great superpower had been defeated emboldened enemies of democracy everywhere.

How valid is the parallel with Iraq and the Middle East today?

Few in the US — even Democratic opponents of the war – dispute seriously that if the US were to pull out of Iraq soon the regional consequences would be devastating.

Just as the vacuum in South-East Asia was filled by an ensemble of enemies, so the Middle East would presumably be the scene of new horrors.

In Iraq itself, the incipient civil war between Shia and Sunni would explode into a full-scale conflagration. Neighbours, with powerful interest in the outcome of that struggle, could be expected to pile in to pursue their own objectives.

It is unlikely that the Sunni governments in Saudi Arabia and Jordan would stand by as their brethren were slaughtered in Baghdad and elsewhere.

Iran could be expected to back the Shia and increase its own leverage inside the country. In the north, the Turks would be likely to regard with alarm the spectacle of a Kurdish-run entity, with worrisome implications for Turkey and its Kurdish minority.

Above all, just as America’s loss in Vietnam to its communist enemies emboldened US opponents and weakened resolve to pursue foreign policy interests, so a defeat to al-Qaeda in the biggest conflict of the post9/11 era would seriously encourage Islamist terrorist groups everywhere.

Of course, critics of the war will argue that these disastrous consequences are either already in place, or will eventually transpire whenever the US leaves Iraq. Delaying that departure will only add to the regional misery in the long term, at the cost of many more US casualties.

The ultimate tragedy for the US is that this political debate seems likely to rage on unresolved for years – just as it did over Vietnam, for years before that war was finally brought to an ignominious close.

1975

LAOS Laos was bombed heavily by the US in attempt to disrupt Viet Cong movements there. After American withdrawal, Soviet-backed guerrillas deposed the royalist Government and installed a client Government of newly united communist Vietnam

THAILAND Thailand absorbed enormous numbers of South-East Asian refugees after the war who had fled the upheaval in their own countries. In 1979 alone, 200,000 Cambodians arrived fleeing the invasion by Vietnam. Fearing such huge numbers could destabilise the Thai monarchy and Government, many have been forcibly repatriated many.

NORTH VIETNAM North Vietnam’s communist government came to control the whole country soon after US withdrawal. The country remains a one-party state in which political opposition is proscribed. Partial free market reforms in 1986 have brought recent economic growth

CAMBODIA Cambodia’s monarchy was overthrown during the Vietnam War and in its aftermath Pol Pot’s Khmer Rouge guerrillas were able to take control. As many as three million Cambodians were killed, tortured or worked to death in his attempt to create an agrarian society. The killing ended when Vietnam invaded in 1978 to be replaced by fighting between occupiers and rebels that lasted through the 1980s

SOUTH VIETNAM South Vietnam’s democratic Government was replaced by the nominally independent Provisional Revolutionary Government of the Republic of South Vietnam. This was controlled directly by North Vietnam, with whom it elected to merge the following year

2007

LEBANON A proxy theatre for the wider conflict, with Iran strengthening Shia Hezbollah in its conflict with Sunnis and Christians

NORTHERN IRAQ Without the US, there is nothing to stop Turkish troops marching into northern Iraq to quell cross-border attacks from Kurdish separatists

JORDAN Would be under pressure to help Sunni cousins in neighbouring Anbar province, setting scene for Iraqi-Arab conflict

IRAN Emboldened by US departure, Iran flexes muscles and battles to ensure a Shia government to its liking in Baghdad. Pours fighters and equipment into Iraq

SAUDI ARABIA/GULF STATES Responds to protect Iraq’s Sunni minority, setting scene for permanent conflict with Iran and within Iraq

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