Washington is numb during a presidential campaign. The oxygen of power drains to the hustings. Blossom droops, restaurants empty, pompous porticos slump as their tenants depart. Even the issue of Iraq, whose subsidies fund more of Washington than they do Baghdad, has left town and gone local.

The one thing known by all three candidates for the presidency (if not just two from today) is that whoever wins must do something painful. He or she must negotiate the terms of an eventual retreat from Iraq, not with the Iraqis but with the American people. Even John McCain, who watched the retreat from Vietnam and swears he will "stay a hundred years in Iraq until peace, stability and democracy" are achieved, will eventually leave, if only under the lash of Congress.

Yet now is not the time to admit it. A war that is unpopular with 60-70% of Americans (depending on the question) is not politically sustainable, however stupefying the cost. But the modalities of its ending are unpredictable and possibly humiliating. Both Hillary Clinton and Barack Obama may call for early withdrawal, at least of "combat troops". But the real paradox of Iraq is that McCain knows he must find a way of leaving, and Clinton and Obama know they must find a way of staying, if only for the time being. For all of them, getting from here to there crosses uncharted territory and none wants to glimpse the map.

Though foreign policy is rarely salient in peacetime elections, Americans have been almost persuaded by their president, George Bush, that they are not at peace. To visit America at present is to be reminded of the continuing trauma of post-9/11, of a nation that craves a cohering substitute psychosis for the lifting of the Soviet menace. It is seen in ubiquitous threat alerts, hysterical airport security, the continued acceptance of Guantánamo Bay and even jibes about public figures not wearing the American flag in their buttonhole. A country in so many ways a kaleidoscope of the world is in many ways so different. Above all it is full of soldiers.

Americans still do not travel abroad, and rely on television news for their knowledge of foreign places, which they continue to regard with bizarre suspicion. Hence a world view is lumped in with defence and security in a collective paranoia. And a candidate's stance on foreign policy is a proxy for his or her character.

To this the candidates must pander. Hence Clinton emphasises her "role" in Kosovo and her "mis-remembered" landing in Bosnia under fire. Obama stresses his links to three world continents and a seminal visit as a young man to Karachi. McCain trumps them by having been tortured by the Vietnamese, a sanctification whose only drawback is that it recalls his age (71).

All must appear trigger happy. McCain may distance himself from the unilateralism of George Bush and remark that Americans must show "a decent respect for the opinions of mankind" (in Bush's America the remark was worth reporting). But his team is penetrated by such neocons as Robert Kagan and John Bolton, on the basis that "if we can't beat him, we can persuade him". The only thing to be said with confidence about McCain is that his position on everything is uncertain.

Desperate not to be outflanked on defence, Clinton said yesterday that she would "totally obliterate" Iran if Iran bombed Israel. Last week she offered an astonishing nuclear-shield guarantee for neighbours of a nuclear Tehran. Obama duly chided her as "Annie Oakley with a gun". Yet he has tended to follow her positions with a ready me-tooism, as on Tibet. He offered to bomb Pakistan terrorist hideouts on the basis that even if that country's President Musharraf "won't act, we will". He wanted two more brigades sent to Afghanistan.

Everywhere is on display the conundrum described in James Sheehan's The Monopoly of Violence, subtitled Why Europeans Hate Going to War. A more realistic title would be Why Americans Love It.

Europeans, writes Sheehan, have tested war to destruction as a way of settling the world's ills and reject it. Electorates now demand "material wellbeing, social stability and economic growth" and have demoted military virtues and the military class to history's dustbin. In modern Europe, "colonial violence seems wasteful, anachronistic and illegitimate ... grandeur no longer an important goal". That is why few Europeans other than Britons will help America in escalating the Afghan conflict. They just do not believe it will work.

To Americans it "must work". The mistakes made by America in Iraq and Afghanistan are seen from Washington as accidents in necessary wars, as they might have been in Britain in the 19th century. Such wars present puzzles to be resolved, tests for weapons systems, trials of strength for Pentagon lobbies, budget barons and thinktanks. And they seem very, very far away.

Enthusiasts for Obama, more plentiful beyond America's shores than within them, regard him as the most plausible candidate to pilot America to a new and more internationalist haven than this. He has spoken of an endgame to America's hostile relations with the Muslim world and dismisses democratic nation-building in Iraq as "a bunch of happy talk". He says simply: "We cannot bend the world to our will."

This may be true, but it is increasingly dangerous for Obama. His handling of foreign policy has been naive and reactive. His weakness is that he seems unknown, not quite American, exotic, elitist, intelligent. He can write his own books, but can he hack his own war?

Hence Clinton's notorious "red-phone-at-3am" advertisement - implying that a black man with a foreign name could not be trusted with the nation's defence - was so lethal, especially her aside that "as far as I know" he is "not a Muslim". It is why, were Obama to emerge from this week's still uncertain events as the Democratic candidate, the smart money in Washington is still on McCain to win a dirty election.

At a distance I continue to find Obama one of the most exciting and potentially able men to run for the American presidency in a generation. His capacity to transform America's self-image and world image is colossal. But to do so he must confront America atavistic love affair with war, and that will be hard.

simon.jenkins@theguardian.com