What didn't happen on August 31, 2013, could someday decide Barack Obama's place in history.

The US president had threatened airstrikes if Syrian leader Bashar al-Assad used chemical weapons against his own people.

When al-Assad crossed Obama's "red line," US fighter jets were at the ready, but the strikes never came. Obama had decided at the last minute against military intervention.

The NATO summit this week in Warsaw will be among Obama's last international meetings. In Washington, many are urging the president, who leaves office in January, not to give up the US role at the forefront of the international community, particularly in Europe, where the shock of Britain's vote to leave the European Union is still raw.

"We are the leader of this alliance. We always have been. And the alliance desperately needs leadership," former top diplomat Nicholas Burns said.

Obama, however, has shown little interest in that role in the past, including at the last NATO summit in 2014. Europe has not been a priority for Obama, who instead has been focused on his "pivot" to Asia and avoiding being drawn into more Middle Eastern conflicts.

Critics accuse Obama of leaving a power vacuum. Despite his self-declared successes like the Iran nuclear deal or rapprochement with Cuba, the Middle East is in chaos with the rise of the Islamic State movement.

Obama, the Nobel Peace Prize laureate, reached an unwelcome milestone in May, having been at war longer than any president before him, including Republican predecessor George W Bush.

Obama was elected in 2008 by a country weary of wars in Iraq and Afghanistan. He promised change, but eight years later US soldiers remain in Afghanistan, even as the mission has officially shifted from combat to assisting Afghan forces.

Obama withdrew US troops in late 2011 from Iraq. Since then, Islamic State forces have seized huge territories in the country and neighbouring Syria, and the US military is conducting strikes nearly daily.

Obama dramatically increased use of drones and has conducted frequent airstrikes against terrorist targets in Libya, Pakistan, Somalia and Yemen. Small bases have proliferated, and Obama has expanded special forces deployment.

"It has been a major criticism of Mr Obama's leadership that he has not explained it to the American people," historian Richard Kohn told dpa. "I think it is evidence that he does not consider himself a war president but a president at war, who inherited wars."

Obama had seen how war could consume a presidency, and his goal instead is "to extract the United States from these wars and to concentrate on a much broader and more domestic agenda," Kohn said.

Obama's government has been calculated about how it describes the military operations.

"War doesn't exist anymore in our official vocabulary," Philip Gordon, Obama's former advisor on the Middle East, told the New York Times.

Hardly a Pentagon press conference goes by without questions about whether special forces in Syria and Iraq are engaged in combat on a mission officially for training and advising local forces.

"The nomenclature becomes important to the administration, to maintain its narrative that we are intervening rather less in a civil war and more in a fight against a terrorist organization that threatens the United States," Kohn said.

Obama's restraint left a vacuum that Russian President Vladimir Putin sought to fill when he deployed Russian jets last year to Syria to prop up al-Assad.

Obama never wanted to intervene in the Syrian civil war, said journalist Jeffrey Goldberg, who interviewed Obama over the course of months for The Atlantic magazine.

The administration has sought to portray unity on its Syria policy, but discord behind the scenes became apparent last month when 51 State Department officials signed an open letter calling for airstrikes against the Syrian regime.

"There's a playbook in Washington that presidents are supposed to follow," Obama told The Atlantic.

"It's a playbook that comes out of the foreign-policy establishment. And the playbook prescribes responses to different events, and these responses tend to be militarized responses. Where America is directly threatened, the playbook works. But the playbook can also be a trap that can lead to bad decisions."

Obama's 2013 decision against airstrikes broke with that playbook, but the new US role remains undefined.

"If you want to be a superpower you're going to have to be a superpower," Obama's former national security advisor James Jones said. "You cannot just pick and choose where you want to be a superpower."

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