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In 1971, when John McCain was starting his fourth year as a prisoner of war in Vietnam, Barack Obama was in Honululu, beginning the fifth grade.
This biographical divergence, the result of age, upbringing and luck, led to a bizarre convergence of focus for the two men in the leadup to last week's Fourth of July celebrations, with one forced to defend the validity of his military experience and the other required to compensate for his own civilian life.
Military service has long been an issue for those running for America's highest office - helping impart an image of courage and resolve for former fighters, and giving a whiff of cowardice to those who stayed home.
But the contrast between veteran and civilian contenders has seldom been as stark as it is in the current cycle, as a war hero is pitted against a man who was only 12 when the last U.S. draft was discontinued.
And so it remains to be seen what role military experience will play in an election face-off between an officer and a gentleman.
Last week, retired general Wesley Clark was criticized for saying, "I don't think getting in a fighter plane and getting shot down is a qualification to become president," a comment that served only to put renewed focus on Mr. McCain's credentials.
At the same time, Mr. Obama spent the week on a patriotism tour, visiting traditionally Republican strongholds, stopping at army bases and delivering an address on the importance of national service, in what many saw as a way to compensate for his own lack of military credentials.
"No one should ever devalue that service, especially for the sake of a political campaign, and that goes for supporters on both sides," Mr. Obama said in response to Mr. Clark's comments. "We must always express our profound gratitude for the service of our men and women in uniform."
The Republican Party is clearly hoping that gratitude will translate into votes for their candidate, whose most distinctive résumé item is his time as a POW and his refusal to accept early release from his Vietnamese captors.
Mr. McCain's television commercials often use footage of the candidate in uniform and refer to his service to the country.
In May, when Mr. Obama questioned the Arizona senator's opposition to a new bill for veterans benefits, the Republican refused to acknowledge a rebuke from someone "who did not feel it was his responsibility to serve our country in uniform."
But having a uniform in the closet is not always a prerequisite for occupying the Oval Office. In the past four general elections, the candidate who has served in an overseas war has been defeated by the candidate who has not.
George W. Bush, who was in the National Guard during Vietnam and never deployed, beat decorated war hero John Kerry in 2004 and in 2000 won the controversial election against Al Gore, who also deployed to the war zone.
Bill Clinton, who avoided the draft, defeated two distinguished service men - Bob Dole and George H.W. Bush, both decorated heroes of the Second World War.
U.S. history has gone through phases of electing ex-military men as well as those with no war record to call their own.
In the aftermath of the Civil War, there was a long period where only veterans of the Northern Army had a chance of political success, with the exception of Grover Cleveland, who paid a substitute to serve for him. But after Theodore Roosevelt, there was a 36-year span during which no U.S. president had a military background, despite the fact that the country was embroiled in two world wars.
"There have been two periods after long, hard wars when it was pretty much assumed that a successful political candidate would have served in those wars," said Henry Mattox, a Korean war veteran and former member of the U.S. foreign service who maintains the website AmericanDiplomacy.org. "Since the Vietnam War, it's gotten a little more complicated."
Because Vietnam was a divisive conflict for the American people, those who fought in it are not guaranteed political support, he said.
There are signs that Mr. McCain's decision to make his Vietnam service central to his campaign could backfire, as it did for Mr. Kerry in 2004. Back then, the Democratic candidate was mercilessly attacked by a group of veterans who questioned his account of service.
More importantly, however, his opposition to the war upon his return fit nicely into his portrayal by Republicans as an indecisive flip-flopper.
Mr. McCain's opinions of the Vietnam War could serve to undermine his position on the current Iraq conflict.
The Arizona senator has steadfastly stuck to his belief that the Vietnam War could have been won, if only the country had committed more troops and greater firepower. Reminded of this, voters could shudder at the thought of a similar commitment to Iraq.
And in terms of votes, there is no indication that his war service is helping Mr. McCain win over the electorate.
During the 2004 presidential election, the Pew Research Centre found that just one in five believed it is very important to learn about a candidate's military service, up only slightly from 19 per cent in 1999.
Richard Kohn, a professor of history, peace, war and defence at the University of North Carolina at Chapel Hill, said there is no correlation between military service and effective leadership.
"We've had terrific presidents who haven't had any military service and we've had abysmal presidents who have been quite famous when they were young and in service," he said.
