The astonishing evaporation of so much of the Iraqi army over the past week gives us one of those rarified moments when even Washington seems at a loss for words.

"That's a bad sign," Missouri Senator Roy Blunt suggested, with supreme understatement, when he was told Iraqis troops had taken to wearing civilian clothes under their uniforms.

Indeed, yes. Sure enough, within days of the lightning advances of the ruthless Sunni jihadists known as ISIS, four of Iraq's 14 divisions were goners, their equipment abandoned, uniforms scattered in and around the northern cities of Mosul and Tikrit.

Since then, it appears that some of the divisions around Baghdad and the predominantly Shia south may prove more resilient, along with the group fighting to retake control of the Baiji oil refinery about 130 kilometres north of the capital.

But as a unified military, Iraq's army seems to have cracked into too many disparate pieces.

"They are crumbling. They're losing confidence in themselves and in their government's ability to win," James Dubik, a retired U.S. general who oversaw much of the training of the new Iraq army in recent years told the Washington Post.

"And the government is losing confidence in them."

The widespread collapse of any army is an amazing spectacle, as it seems to run against nature and spreads like fury down a country's spine, connecting one terrified city to another.

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A fighter with the militant Islamic State of Iraq and Syria stands guard at a checkpoint near the city of Baiji, north of Baghdad, on Thursday. Behind him is what appears to be a confiscated Iraqi military vehicle. (Reuters)

Older Americans don't have to be reminded of the risks, nor what might soon present itself. They've seen an army collapse before in circumstances that awkwardly mirror Iraq in recent days.

In the spring of 1975, South Vietnam collapsed within an astonishing two months at the very time the U.S. was trying desperately to move beyond its sorry eight-year-long war there.

America had already withdrawn most of its troops following the 1973 ceasefire agreement, but had continued to train the southern government extensively and expensively, all to no avail.

The South Vietnamese army disintegrated along its north-south corridors before a Viet Cong offensive that saw the last U.S. advisers helicoptered off the embassy building in Saigon.

Little to show

Flash forward almost 40 years, Obama announced yesterday he would be sending another approximately 300 military "advisers" to Iraq, to help its army, but primarily, the president said, to safeguard U.S. embassy officials and other Americans in Baghdad in the event the Islamic State of Iraq and Syria militants advance further.

There would be no redeployment of U.S. troops; targeted airstrikes were mentioned only as a possibility.

After billions of dollars were spent arming and training South Vietnam, it still turned out the officer corps there was corrupt, and political leadership ineffective.

In Iraq's case, the U.S. spending frenzy was even more pronounced, and there seems equally little to show for it.

Washington managed to spend $25 billion just on training Iraq's new army over the years 2003-2010, while many more billions were spent by the Iraq government on top U.S. gear like M1tanks, Apache helicopters, even F-16 fighter jets.

Much of the high-tech equipment fell into disrepair after U.S. advisers began pulling out in 2010. And much of the remaining equipment is now falling into jihadist hands, which are already well supplied by lucrative militia fighting in Syria.

By the end of last year, U.S. studies reported, up to 180,000 Iraqis had died in the years since the invasion, as well as almost 4,500 Americans.

Best estimates of the economic costs so far is $1.7 trillion, by Reuters, with an additional $490 billion in benefits owed to war veterans.

Now, if Iraq collapses further into full sectarian civil war between Shia and Sunnis, as many expect, it will seem as if no humiliation is to be spared the U.S. for its wildly reckless invasion of 2003.

The best and brightest?

How can one even begin to explain such a fiasco?

Famously America's "best and the brightest" diplomats and generals brought about the Vietnam quagmire in the 1960s. Iraq is often put down to Republican nation-building hubris in the wake of 9/11.

Certainly, following the invasion, one blueprint after another to develop Iraqi security forces have been treated like profoundly radioactive dust.

Should Iraq's moderate Sunnis resist active military service under the rabidly pro-Shia Prime Minister Nouri al-Maliki, it is difficult to see how Iraq can be held together; and equally hard to see how large chunks of the country will not become a haven for worldwide terrorist operations, rather like Afghanistan was before 9/11.

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Barack Obama speaks about the situation in Iraq in the briefing room of the White House on Thursday. No commitment of U.S. troops but prepared to take "targeted military action" if necessary. (Kevin Lamarque / Reuters)

The U.S. has pleaded with al-Maliki to reach out to fellow Sunnis and Kurds, but he remains largely intransigent, and so the sense of spiraling ever downward resumes its horrifying course.

The other reality is that there were fatal flaws with Iraq's supposedly rebuilt, 250,000-strong military that were already obvious to observers by at least 2010.

That year the International Crisis Group published a 50-page report in which it criticized the Iraq army's worsening "cronyism, bribery, kickbacks, extortion."

Units were being infiltrated for political purposes, while others "suffered high desertion rates, abysmal readiness and infiltration by insurgents, militia and criminals," the ICG report said.

President Obama has been strongly criticized for having pulled most military trainers from Iraq by the final pullout at the end of 2011, so determined was he to break free of what he called George W. Bush's "dumb" war.

But it has proved impossible to escape the scourge of history.

In this case, shortly after the 2003 invasion, the U.S. committed one of the most disastrous fumbles in American foreign policy history when, without serious study, it simply disbanded the entire 400,000-man Iraqi military, along with much of the civil service.

It was part of a sequence of decisions, beginning with the phony rationale for the invasion itself, that turned Iraq into a house of horrors.

Its society was profoundly shaken and demoralized as tens of thousands of humiliated former soldiers were abruptly thrown out of work, becoming easy recruits for armed militias. The battle for the cities was on.

The decision to rush in to rebuild an army from scratch was then hobbled by the "dysfunctional U.S. approach to institution-building resulting from lack of knowledge," said the ICG report.

And the world will continue to pay a frightful penalty for that lack of awareness.

I still shudder when I think of the Lebanese civil war in the 1970s and '80s, which lasted a dozen years and involved many of the same forces in bloody conflict. This one looks to be very much bigger and even more vile.