Image copyright AP Image caption Jeb Bush has struggled to articulate what he would have done over Iraq

On both sides of the Atlantic, the Iraq war continues to cast a diffuse shadow from which politicians are struggling to escape.

It falls upon the fight for the Republican presidential nomination, Hillary Clinton's White House quest and the debate over the future direction of the Labour Party in Britain.

Even freshman politicians, like Florida Senator Marco Rubio, who was three years shy of heading to Washington when the statue of Saddam Hussein was toppled in Baghdad's Firdos Square in April 2003, have been touched by its gloom.

Seeking to follow his brother into the White House, it was inevitable that Jeb Bush would be quizzed about the decision to go to war.

What's been surprising, astonishing even, is his fumbling response. After all, he and his team must surely have war-gamed such a predictable line of inquiry.

Over the course of four days earlier this month, he came up with four different iterations of his policy.

To start with, he told an interviewer on Fox News that he would have authorised the invasion, and then added buoyantly: "So just for the news flash to the world, if they're trying to find places where there's big space between me and my brother, this might not be one of them."

By the end of the week, he had flip-flopped, that cardinal sin of candidates seeking the presidency. "I would not have engaged," he now ventured, prompting much ridicule in the press. "I would not have gone into Iraq."

Geopolitical superstorm

More unexpectedly, every Republican candidate is being pressed on whether they, as president, would have invaded Iraq.

Interrogated on the war question, again on Fox News, Mr Rubio offered what sounded like a reasonable formulation - that presidents never get the luxury of making decisions with the benefit of hindsight.

Image copyright AP Image caption Senator Marco Rubio has had a torrid time in the news studios over Iraq

Yet that did not prevent him from getting a torrid grilling from host Chris Wallace, which damaged his campaign.

Even as the world is being buffeted by a geopolitical superstorm, with wars in Syria, Iraq, Ukraine, Libya and elsewhere, the prime question for Republicans has been a historical hypothetical.

With the race stuck in a sort of time warp, a war that started 12 years ago has become a litmus test of their foreign policy credentials.

"Knowing what we know now, would you have authorised the invasion of Iraq?"

Jeb Bush (11 May): "I would have. And so would have Hillary Clinton, just to remind everybody. And so would almost everybody that was confronted with the intelligence they got."

Jeb Bush (12 May): "I interpreted the question wrong, I guess. I was talking about, given what people knew then, would you have done it, rather than knowing what we know now. And knowing what we know now, you know, clearly there were mistakes." But would he have invaded? "I don't know what that decision would've been."

Jeb Bush (13 May): "What we want to be focusing on are what are the lessons learned… I think the focus should be on that, on the future."

Jeb Bush (14 May): "If we're all supposed to answer hypothetical questions - knowing what we know now, what would you have done - I would have not engaged. I would not have gone into Iraq."

For Mrs Clinton, Iraq was a much larger problem in 2008 than it is today. Back then, Barack Obama, an anti-war candidate, hammered her for voting in the Senate to authorise military action.

In her 2012 memoir, Hard Choices, she sought to cauterise the wound by admitting she had erred. "I thought I had acted in good faith and made the best decision I could with the information I had," she wrote.

"And I wasn't alone in getting it wrong. But I still got it wrong. Plain and simple."

Image copyright AP Image caption Hillary Clinton: "Some suspect that she remains, at heart, a neo-liberal interventionist"

To this day, though, there's lingering mistrust over her support for the war, especially on the left of the party. It amplifies the criticism that she sails with the prevailing political winds, which in October 2002, when the Senate voted, were at the back of George W Bush (29 of the 50 Senate Democrats voted for the war).

It also suggests that her instincts are hawkish - far more interventionist than Mr Obama's, and far more interventionist than America's war-weary electorate.

Writing about her "Iraq dilemma" in the New Yorker, John Cassidy noted: "Based on her history, some analysts suspect that she remains, at heart, a neo-liberal interventionist, a la Tony Blair."

Blairism and Iraq

That the words "a la Tony Blair" have become journalistic shorthand for a disputed foreign policy is indicative of how the Iraq war still impacts on Labour politics in Britain.

Though it would be ridiculous to argue that the 2003 invasion frames the leadership context - each of the four contenders supported the decision to go to war - it does contribute to the subtext.

The war makes it harder for those with a Blairite agenda, like Liz Kendall, to make their case, for the simple reason that Blairism and Iraq have become conjoined.

Image copyright PA Image caption Labour leadership frontrunner Andy Burnham has distanced himself from Tony Blair

When many Labour party members think of their former leader, they recall his kinship with George W Bush rather than his affinity with the British electorate, which gave him three consecutive general election victories.

His last victory came in 2005, by which time it was clear there were no weapons of mass destruction in Iraq. But the post-war mess in the country contributed to Labour jettisoning his successful electoral formula, targeted at Middle England, and moving leftwards.

Labour leadership contenders in their own words

Andy Burnham: "If we are asking about what Labour is for now then it's about the ­aspiration of people being able to pass on what they've worked so hard for... I'm not tangled up in the Blair/Brown stuff. I'm free of that."

Liz Kendall: "The major problem we have got is that, fundamentally, people did not trust us on the economy. Above all, we have not set out a compelling vision for the future. I am a moderniser true to our values."

Mary Creagh: "I want to earn back the trust that Middle England has lost in the Labour Party. We forgot the hard-learned lessons of our last three election victories; that to win elections a party needs to offer hope."

Yvette Cooper: "We can't repeat the narrow approach of the last five years. But nor should we think the answer is to swallow the Tory manifesto instead."

Mr Blair's toxicity partly explained why David Miliband, his former policy director and protege, lost the Labour leadership in 2010, and also why his brother Ed, a devout Brownite, steered the party away from New Labour.

Andy Burnham, the frontrunner in the present contest, used to describe himself as a Blairite but has distanced himself from the former leader over the past five years.

Writing in the Guardian last week, Jonathan Freedland argued that Labour needed to get over its Blair problem. Needless to say, the Blair problem is a catch-all that includes Iraq.

Not insignificantly perhaps, the British parties that have fared best in recent years, the Conservatives, the Scottish Nationalist Party and UKIP, have been largely untroubled by the politics of Iraq.

Image copyright EPA Image caption The conflict in Iraq is set to intensify as Shia militias and others battle Islamic State militants

When will the shadow of Iraq be lifted? Not any time soon it seems, especially when the conflict in the country is set to intensify rather than abate.

It is also worth remembering that Vietnam continued to shape presidential politics more than three decades after that last American chopper took off from the US embassy's rooftop in Saigon.

Vietnam displaced

Bill Clinton ran into problems in 1992 over allegations of draft dodging. Mr Bush's decision to serve with the Texan Air National Guard during the Vietnam War dogged him both in 2000 and 2004.

That year's Democratic presidential nominee, John Kerry, a recipient of the Purple Heart, was unfairly criticised - or "swiftboated" to use the neologism spawned by the controversy - over his service and anti-war activity following his return home.

The reason why the Vietnam syndrome no longer looms as large is not purely generational (Mr Obama was aged six at the time of the 1968 Tet Offensive). It is also because Iraq has displaced Vietnam in the national psyche.

Maybe this electoral cycle will be the last time the Iraq question will be asked with quite so much urgency and insistence.

But as the repercussions of that war continue to be felt, not just in Iraq but globally as well because of America's post-Iraq military wariness, it looks set to remain relevant for many years to come.