Hagel, Obama’s pick for defence secretary and Kerry, his secretary of state nominee, were both wounded in Southeast Asia, and both, back home, evolved into sceptics of US adventurism overseas.

Later, both also saw worldviews, forged under enemy fire, clash with the new US militarism of the post-September 11 world, and until Obama came calling, were marooned in political backwaters.

Democratic Senator Kerry, 69, had lost a White House bid and was cruising towards retirement. Republican ex-senator Hagel, 66, was reviled in his party for Iraq war apostasy.

“These will be the final veterans of Vietnam with responsibility and in power,” said Julian Zelizer, a Princeton University historian.

“Both in some ways reflect some of the lessons of Vietnam and see the world through that prism.”

Sean Kay, professor of international relations at Ohio Wesleyan University, said both men could change the US reflex to use the military as the “lead spear” of foreign policy.

“There is [now] a strategic rationale for a worldview that shows a powerful and confident America, but also one that is guided by restraint in terms of how and when it uses its military power.”

With Hagel facing a tough Senate confirmation, over past remarks on Iran, gays and Israel, some political analysts wonder why he was chosen at all.

But though a staunch conservative, Hagel has long shared a strategic meeting of the minds with liberal Obama, the anti-Iraq war candidate who raged against “dumb” wars on the way to winning the 2008 election.

While naval lieutenant Kerry returned from Vietnam to become an anti-war talisman, asking how many more men needed to die for a “mistake,” infantryman Hagel took longer to sour on a war that killed 58,000 Americans.

But he told C-SPAN television in 2005 that Vietnam showed: it’s easy to get into war, it’s not easy to get out.”

Obama struck a similar note on Monday.

“[Hagel] understands that sending young Americans to fight and bleed in the dirt and mud, that’s something we only do when it’s absolutely necessary.”

Hagel, like Kerry, voted in 2002 to authorise president George W. Bush to invade Iraq.

But by 2005, he warned the war was being lost. He later bemoaned the US troop surge as the biggest blunder since Vietnam and in 2007 was against putting more American lives into a “grinder”.

His stance on the surge, which many Republicans view as enabling a dignified US exit from Iraq, may partly explain antipathy to Hagel in his own party, in which he was never personally popular.

While Hagel and Kerry, like Obama, back a foreign policy based on alliances, diplomacy and international institutions, their selection is not a hint the president has suddenly sworn off the military.

Obama, while ending land wars, has shown a penchant for lethal force – in Libya, in the raid to kill Osama bin Laden, in the troop surge in Afghanistan, and in escalating drone attacks on terror suspects in Pakistan and elsewhere.

The president has said “yes, to pulling back in the sense of a military footprint and yes to accepting a diminished role for the US which is somewhat less of a hegemonic one,” said prominent US foreign policy author James Mann.

“But he has also conveyed the idea that the use of force occasionally is strongly in America’s interest.”

But with Hagel and Kerry, Obama was hardly stocking his cabinet with hawks, as he contemplates whether to risk a third US war in the Muslim world and strike at Iran’s nuclear programme, if diplomacy fails.

Hagel has blasted Iran war talk as “dangerous,” outraging conservatives who fear he could undermine US threats of force.

“It’s a signal you’re sending to Iran at the worst possible time,” Senate Republican hawk Lindsey Graham said.

Still, conservative claims that Hagel is out of the “mainstream” ring hollow, given that polls show most Americans oppose war in Iran.

And the fight over his nomination may reflect a joust between realists like Hagel and the president, and still vocal neo-conservatives.

“In some ways this is not a fight against Obama, but a much broader fight within the Republican Party over the direction of foreign policy,” said Mann.