Ten years after the US-led invasion of Iraq, the searing memory of what became a deeply unpopular war has made Washington policy makers reluctant to use even limited force in Syria or Iran.

After launching a war to oust Saddam Hussein, the United States faces another isolated Arab Baathist dictator accused, in his turn, of hoarding chemical weapons, supporting terror groups and brutalising his own people.

But two years into the revolt against Syria's Bashar al-Assad -- a conflict that has killed more than 70,000 people -- even the most hawkish voices in Washington are merely calling for arming the rebels or enforcing a no-fly zone.

And despite years of fruitless negotiations over Iran's controversial nuclear program, both candidates in last year's US election spoke of military force as a last resort to prevent Tehran from getting an atomic bomb.

Washington seems to have heeded the parting quip of former defense secretary Robert Gates, who suggested in 2011 that anyone advocating a land war in the Middle East or Asia should "have his head examined."

With a still-sputtering economy and spiraling debt, US President Barack Obama has repeatedly called for nation-building at home, vowing to withdraw from Afghanistan next year after pulling out of Iraq in 2011.

The "shock and awe" of the Iraq invasion -- a media-blitzed display of American power aimed at transforming the Middle East -- has been replaced by the unseen work of aerial drones buzzing over the region's backwaters.

And in Libya, Washington was content to "lead from behind" as it took part in a UN-authorised and Arab-backed NATO intervention involving no US ground troops -- a far cry from the "coalition of the willing" that invaded Iraq.

"One has to be very careful about jumping into situations you don't understand, whose unintended consequences can be very, very surprising," says Christopher Hill, who served as US ambassador to Iraq from 2009-2010.

But he fears policy makers "may have over-learned their lessons, because I think we do need to be more diplomatically engaged in Syria's future."

Many of the neo-conservatives who dominated the first term of George W. Bush's presidency and championed the invasion of Iraq have pressed for more strident action in Syria.

But this has drawn comparisons, not with the 2003 war, but with the aftermath of the 1990 Gulf War, when the United States stood by as Saddam brutally crushed an uprising Washington had helped to incite.

And even the most strident advocates of American force have stopped well short of calling for boots on the ground.

"It is perfectly understandable why the Obama administration wants to do nothing that would lead to a repetition of the invasion of Iraq," former deputy defense secretary Paul Wolfowitz, one of the most ardent advocates of the 2003 invasion, wrote in the Wall Street Journal in January.

"But no one is arguing for any such thing," he added.

Washington's reluctance to intervene in the region is not without precedent -- in the aftermath of the Vietnam War Richard Nixon elected to arm US allies instead of intervening on their behalf to prevent the spread of communism.

In the Middle East, that meant transforming the Shah's Iran into a regional policeman armed with billions of dollars of military aid, a policy left in shambles by the 1979 revolution that overthrew the autocratic monarch.

As the United States navigates another period of intense regional turmoil, this one unleashed by the 2011 Arab Spring, demons left over from Iraq could make it difficult for Washington to influence events in a positive way.

"The Iraq experience has led the US to become cautious to a fault," says Steven Heydemann, a senior advisor on Middle East initiatives at the US Institute of Peace.

While advocates of intervention in Syria exaggerate the strategic benefits of eliminating a key Iranian ally, he says, "critics of intervention tend to exaggerate the risks, talking about 'mission creep' as if it were inevitable."

He says Washington should seek a "middle ground" by supporting Syria's rebels -- as it did last month, by offering food and medical aid but no arms -- without intervening militarily, which would be unpopular in both countries.

But Heydemann admits that "so much resentment and anger has built up over the hesitancy and timidity of US policy that it will take a lot to persuade forces on the ground that the US is now serious."