Washington - Vietnam has been the ghostly presence in the room as the US went to war in Iraq - a finger-wagging reminder of past failure. US Senator Edward Kennedy, the lion of US liberalism, has called the war "George Bush's Vietnam."
The misleading information about Iraq's weapons of mass destruction used to justify the war has reminded some of the 1964 Gulf of Tonkin resolution that gave Congressional approval to escalating the US military presence in Vietnam.
Afterwards, it appeared that the alleged attacks on US ships in the gulf used to justify the escalation had in fact not occurred - just as it turns out that Saddam Hussein had no unconventional weapons.
By 1968, the US had 500,000 troops in Vietnam. When the Vietcong in the south and the North Vietnamese army launched their first coordinated strategy, the Tet Offensive, early that year, US forces started suffering huge losses.
Then US president Lyndon Johnson asked for another 300,000 troops, provoking massive debate in Washington and triggering widespread campus protests against the draft.
By March, Johnson was so heavily under political fire that he dropped out of the presidential race to pursue a peace initiative - an announcement that stunned the country. The badly splintered anti- war movement and the Democratic Party lost their only hope for the White House - Bobby Kennedy - to an assassin's bullet in June 1968 and Republican Richard Nixon won the election.
It took another seven years for the US to withdraw, but the 1968 Tet Offensive is seen by many historians as the watershed that put the US on the path to failure.
But no other event in US military history to that time so shook the country and the world as the My Lai massacre on March 16, 1968.
For an entire generation of Americans, Europeans and Asians, My Lai stood for the image of the ugly American, resonating with the same stinging shame that Abu Ghraib and Guantanamo carry for the current generation.
The soldiers of the 1st Battalion, 11th US Army Brigade, arrived in the morning in the village of My Lai, in South Vietnam's highlands.
"The orders were to shoot anything that moved," one US army officer told journalist Seymour Hersh, the investigative journalist who broke the story a year later.
When the brigade left three hours later, My Lai was levelled. Blood-soaked bodies covered the ground - women, children, old men, dogs.
The number of dead ranged from 347 to 504, victims of a Charlie Company gone amok in blood lust. Not one shot had been fired at the infantry.
It would be another year and a half before the US public learned the truth despite a cover-up by the US Army. The revelations fed outrage across the US and world.
But it wouldn't be until April 1975 that the US finally pulled out of Saigon, boarding helicopters from the rooftops.
The superpower had suffered a humiliating defeat, outfought in a guerrilla war on the Cold War front by an enemy with little technology.
During 10 years of war, the lopsided death toll was 58,000 US soldiers versus 2 to 4 million Vietnamese.
The deep psychological wounds of Vietnam are still raw. The war and the defeat imprinted itself on the collective US psyche. Dozens of films and hundreds of books continue to plough the ground.
Every US military engagement abroad is accompanied by the warning, "No more Vietnams!" In the first Gulf War in 1990, it was widely reported that US commanders sent their soldiers into battle with the orders, "No My Lais - you hear?"
But in the second Iraq war, the worst fears have come to pass, with images of humiliation by US soldiers of Iraqi prisoners at Abu Ghraib, stories of the massacre in Haditha, and the allegations of torture of terrorist suspects at Guantanamo.
"The shame of Haditha" would further erode the image of US military forces, Time magazine warned about the November 19, 2005, massacre of 24 Iraqis by US marines. They were retaliating for the death of a marine lance corporal in a roadside bombing.
As Iraq sank further into chaos and the number of dead climbed - an estimated 60,000 civilians, 4,000 US soldiers - comparisons with Vietnam were unavoidable.
Only one officer was convicted for the murder of 22 My Lai civilians - Lieutenant William Calley, who served three and a half years in jail.
In the Haditha case, three officers were reprimanded for faulty responses while Staff Sergeant Frank D Wuterich faces charges of unpremeditated murder of 18 civilians.
Higher ranking officers have not been convicted in either case.
Last summer, historians were outraged by US President George W Bush's comparison of Vietnam to Iraq. Bush likened the dangers of withdrawing from Iraq to the "killing fields" that followed the US withdrawal from Vietnam.
The 2008 release of Pinkville, a new film about My Lai by director Oliver Stone, will no doubt bring up more comparisons to current US military actions.
