He defended America's right to wage war provided it was “morally justified” and acknowledged the paradox that he was being honoured a week after ordering 30,000 extra troops to Afghanistan.

“War is sometimes necessary,” the commander-in-chief, then fighting wars in Iraq and Afghanistan, told the gathering in Oslo.

“Our actions matter and can bend history in the direction of justice,” he said, warning that “consequences” must back diplomacy to beat repression.

But if the wars in Iraq and Afghanistan were legacies of his predecessor, George W. Bush, Obama will on Friday hand over several thousand US combat troops overseas to his successor, Donald Trump.

Norway's Nobel committee — heavily criticised for choosing a man yet to prove his worth — at the time praised Obama “for his extraordinary efforts to strengthen international diplomacy and cooperation between peoples”.

Seven years later, Obama steps down “neither a pacifist, nor a warmonger” says Joseph Bahout, a visiting scholar at the Carnegie Endowment for International Peace research centre in Washington.

Red line

“He is a realist with a slight penchant for moralising, but he's not an idealist,” he added, calling Obama a proponent of one strand of American foreign policy doctrine.

The attitude that America should no longer be an automatic or zealous policeman of the world puts Obama closer to Trump than the more interventionist approach of George W. Bush or even Bill Clinton.

“The Nobel Peace Prize did not influence his (overseas) action,” said Bahout, who believes Obama was governed by “instinct” in not dragging America into military conflicts.

Faced with the scale of the tragedy in Syria, which has killed more than 310,000 people and displaced millions since 2011, Obama has spoken of feeling responsible even if the window for intervention was limited.

“There hasn't been probably a week that's gone by in which I haven't reexamined some of the underlying premises around how we're dealing with the situation in Syria,” he said last September.

But he told CBS News in an interview broadcast last weekend that he did not regret his famous “red line” speech about Syria's use of chemical weapons — criticised by many of his opponents as a failure.

Speaking about possible US military action, he warned Syrian President Bashar al-Assad in August 2012 that “seeing a whole bunch of chemical weapons moving around or being utilised” would be a “red line”. In 2013, the Syrian military used chemical weapons in an attack on rebel-controlled areas of Damascus, killing nearly 1,500 civilians, more than 400 of them children.

Obama doctrine

After a video caused outrage around the world, the US, Britain and France prepared for imminent air strikes but Washington and London pulled back.

The US instead reached an agreement with Russia to dismantle Syria's chemical arsenal. But Obama could not keep out of the war completely.

In September 2014 he forged an international bombing coalition to attack the militant Islamic State (IS) group and other radical militant units in Syria while pursuing, in vain, a political solution.

But Obama's refusal to put US boots on the ground, apart from several hundred special forces, helped give Russia the military and diplomatic upper hand.

Amy Greene, a researcher on US foreign policy at Sciences Po in Paris, sees no link between Obama's Peace Prize and lack of intervention in Syria.

“Obama wasn't convinced that troops on the ground would improve the situation. It was a head of state's responsibility not to unleash Iraq take two,” Greene told AFP.

But under Obama, the United States has drastically stepped up its use of drone strikes in undeclared wars in Pakistan, Somalia and Yemen, killing 2,581 “combatants” since 2009, according to a US intelligence tally in July.

“We can be sorry that he did not close Guantanamo Bay and that he used a lot of drones but this is a president who has acted according to his principles and his doctrine,” said Greene. -AFP