If, and it is still not certain, Americans have selected John McCain and Barack Obama to contest for the presidency this November, there is one unequivocal and clear benefit. A McCain-Obama fight would place the war on terror and the war in Iraq at the dead centre of the election campaign.

Seven years after 9/11, five years into the Iraq occupation, that’s surely as it should be. And the added good news is that both represent two different, distinct but also honourable and sane positions. Obama opposed the Iraq war in 2003 as strategically dumb and fundamentally misconceived. McCain supported it conceptually but sharply criticised its subsequent execution.

Now, McCain pledges to stay in Iraq for a hundred years if necessary and claims that the surge has altered the cost-benefit analysis. Obama argues that the surge has not solved the fundamental problem and promises to begin withdrawing troops next year. Unlike Hillary Clinton, and unlike this columnist, neither man has actually changed his fundamental position. And so there is now a path for Americans either to fully own this misbegotten Mesopotamian adventure or fully disown it.

Who should have the edge? Only a fool would predict given the twists and turns of the past few years. But the differences going forwards may be more nuanced than some now imagine, and both McCain and Obama are somewhat more complicated figures on the question than the headlines would have you believe. At this point, it seems to me, Obama has a clear advantage. But that advantage is premised on events. And events, as we have seen, can surprise us.

Obama famously opposed the war from the start. But what made his opposition so striking in retrospect was that it wasn’t suffused with hatred of President Bush or leftist rhetoric. It was a realist critique of what he called a “dumb war”. It was an empirical not an ideological judgment. And precisely because it was an empirical judgment, it was not fixed in granite.

The Clinton campaign has described the notion of Obama’s unwavering opposition to the war as a “fairy tale” in order to elide the differences with Senator Clinton’s shifts on Iraq since 2002. That’s unfair. But Obama has been more calculating during the occupation than some now seem to believe – and it says good things about him that he has.

Michael Crowley in The New Republic pointed out some of this last week. Obama was not driven by an absolute sense of certainty in opposing the Iraq war after it started. In his campaign book, The Audacity of Hope, he even conceded that after the fall of Baghdad, as Bush was preening on that infamous aircraft carrier, “I began to suspect that I might have been wrong.”

In April 2003, Obama again struck a sceptical and not a dogmatic note, saying that Bush “is riding high on the whole Iraq situation for the moment, but . . . the jury is still out”. In his breakthrough 2004 convention speech, Obama said: “There are patriots who opposed the war in Iraq and patriots who supported it.” Those aren’t the words of an inflexible peacenik. More pertinently, he consistently said that as someone then outside the loop on Senate intelligence, he wasn’t in a position in 2003 to second-guess Clinton, John Kerry or even John Edwards. “I’m always careful to say that I was not in the Senate, so perhaps the reason I thought [the war] was such a bad idea was that I didn’t have the benefit of US intelligence,” he said in October 2006. “And, for those that did, it might have led to a different set of choices.”

In the end, his antiwar stance was about as consistent as any serious politician’s should be. But it was a prudential judgment on his part, and he always acknowledged it as such. And if it was prudential, it is not unreasonable to infer that it could change again. War is a dynamic process. If circumstances change, if the battlefield shifts, a politician with Obama’s record could adjust.

Obama has already conceded what any future president must: he cannot know future events or contingencies and so cannot categorically say he would withdraw instantly from Iraq upon assuming the presidency next year. His is not a faith-based policy. Which means it can change.

The same should be said about McCain. Again, the great relief about an Obama-McCain match-up is that it helps puncture the faith-based rigidity of the Bush era. They both acknowledge inconvenient facts, which is why, I believe, they have both emerged as popular figures in the wake of the Bush years.

And so McCain never believed that democracy would spring newborn from the chaos of the “stuff-happens” Cheney-Rumsfeld occupation of Iraq. He was a military man and knew that toppling Saddam might take only 150,000 troops, but occupying a ravaged, post-totalitarian, religiously divided non-country required more boots on the ground than Rumsfeld’s “transformational” ideology would allow for.

McCain was not responsible for the change in strategy that pushed Bush to appoint General Petraeus and embark on a sane counter-insurgency strategy. The 2006 congressional elections (in which Republicans took a hit) were. But he deserves credit for championing Petraeus in the spring of 2007 as, in his judgment, the least worst option at hand.

But he is not a utopian. He is against setting a date for withdrawal from Iraq. But he is not against withdrawal as such. In saying that he is prepared to keep US troops in Iraq for a hundred years, he has insisted that he didn’t mean fighting a war for a hundred years. He meant a token force, not in combat, in the manner of the troops still in Germany.

That implies there is a point at which he could say that facts on the ground suggest withdrawal. In fact, his passionate defence of the surge’s success may actually give him more leeway for withdrawal from Iraq as president. After all, if the surge has worked in routing Al-Qaeda, haven’t US goals been met? And morally, the surge goes a long way to responding to the critique: you break it, you own it. Yes, the US broke Iraq. But isn’t the expenditure of this much life and treasure after five years sufficient to argue that it is now up to the Iraqis to solve this problem, not the US?

McCain is not known for infinite patience with duplicitous politicians. You can easily see a scenario in which McCain blows up and washes his hands of the dysfunction in Baghdad. And were he to do so, an awfully large number of Republican voters would be right behind him.

McCain’s long-term record, moreover, has not been uniformly interventionist. In the 1990s, he was an early sceptic of intervention in the Balkans, before being persuaded that genocide had to be prevented and that air power could achieve it in consort with a relatively small allied ground force. In the first Gulf war, like Dick Cheney, he was very concerned about a long and brutal ground campaign that could chew up America’s military. As a veteran of Vietnam, McCain is not unaware of the dangers of quagmires and the need to retain overall military readiness. Among his confidants are the so called “realists” Henry Kissinger and Brent Scowcroft.

McCain, in short, is no more a neoconservative ideologue than Obama is an unreconstructed peacenik. They are both gifted politicians, and like all gifted politicians, they know how to respond to reality. The war debate will be critical to this campaign. But anyone who says they can see its exact parameters is deluding himself.

The good news is that their shared empiricism makes an honest debate finally possible. Even better, at some point, a clear decision will have to be made: double down or get out. We may just be surprised who does the latter.

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