WASHINGTON – In a world scarred by war and threats of still more of it, Barack Obama clings in the final quarter of his presidency to the hope that his lasting legacy will be as a peacemaker.

Despite having already received a surprise Nobel Peace Prize, given essentially on the hope that he would merit it, Obama continues to press on. The deal struck with Iran, ostensibly to cut off its path to achieving a nuclear bomb, is more a signal of intent than an achievement.

Heavy work remains ahead, first to keep Congress from scuttling the deal. Then, just as significantly, Obama needs to convince the American people and the world at large that the nuclear genie can be kept in its bottle through verification of Iran’s compliance with the terms of the deal.

The duty to “trust but verify (to quote Ronald Reagan’s old mantra) lies ahead, not only for months but years and decades. In terms of the Obama legacy, the proof of the undertaking will not come until long after he leaves office.

After enduring a long period of frustration at the hands of the opposition throughout his first term, and well into his second, Obama in this season has encountered some more hopeful skies, even as the wars he inherited have persisted and others have burst forth.

Yet one goal has remained constant: to get the United States “off a permanent war footing” and instead to maintain peace through effective collective action among the nations of the world. It seems almost as though Obama yearns for the early, optimistic days at the end of World War II, when the United Nations was born from the ashes.

This president’s obvious abhorrence of the notion of America as the world’s policeman has not prevented him from selectively answering the world’s alarms. His responses, though, often have been tentative or halfway measures that complicate or even undercut his actions and decisions.

In striving to cope with the continuing aftermath of the wars in Iraq and Afghanistan and the emerging new crises elsewhere in the volatile Middle East, Obama continues to insist that there be no more American “boots on the ground.” It seems a militarily unrealistic condition, especially to experienced military hands.

But the president clings to the view that deploying such troops not only amounts to a temporary band-aid but also undercuts the will of indigenous forces to fight and win their own battles on their own home ground.

This point of view makes an easy target for political and ideological critics, who cast Obama as soft or naive. It invites another round of fierce and emotional political combat from Republican hardliners against the new deal designed to check Iran’s threat to become a member of the nuclear weapons club.

In the earlier efforts of Presidents John Kennedy and Ronald Reagan to strike arms-control deals with the Soviet Union, critics cited its past deceptions. Similarly, critics of the Iran deal argue that the country’s regime has often demonstrated its untrustworthiness and can’t be trusted now.

But Obama, like those two American predecessors, has judged the risk worth taking. He appears particularly motivated by his determination to turn the United States off the course onto which it veered in 2003 with George W. Bush’s elective invasion of Iraq.

Not surprisingly, Obama’s experience in the Oval Office has driven home to him how his lofty domestic objectives as a young and idealistic president became casualties of the wars he inherited. As his tenure as president winds down, this latest deal with Iran gives him a chance to add genuine substance to the Nobel Peace Prize he received back in 2009.