On March 28, 2011, Barack Obama stood behind a presidential podium at the National Defense University and addressed the nation. His ostensible topic was Libya, and his ostensible purpose was to explain his decision to intervene there. And over the course of his 27-minute address, he did this.

Muammar Qaddafi was poised to attack his own citizens. “His forces,” Obama said, “continued their advance, bearing down on the city of Benghazi, home to nearly 700,000 men, women, and children who sought their freedom from fear.” If the United States and its allies had waited, he added, Benghazi could have suffered “a massacre that would have reverberated across the region and stained the conscience of the world.”

Obama used the occasion, though, to speak on themes much grander than the conflict in Libya at that moment in the spring of his third year in office. He used it to describe his understanding of America’s role in the world throughout history. “For generations,” he said, “the United States of America has played a unique role as an anchor of global security and as an advocate for human freedom. Mindful of the risks and costs of military action, we are naturally reluctant to use force to solve the world’s many challenges. But when our interests and values are at stake, we have a responsibility to act.”

Doing nothing, he argued forcefully, was not an option for a nation as great as ours:

A betrayal of who we are. Those words echoed in our ears this week as we watched with horror the slow-motion slaughter in Aleppo, Syria. For months, the civilians who remained there have been waiting and expecting to die. And death has found them by the thousands. Some died in attacks launched by the jihadists who have long held the eastern part of the war-torn city. Others perished in barrel bombings conducted by their own government, which has targeted hospitals, schools, and apartment buildings.

Charred buildings sit abandoned. Many streets are impassable, cluttered with rubble, burned-out cars, and, at times, bodies. Clarissa Ward, a reporter covering Syria for CNN, said: “This is actually hell. This is what hell feels like, and there is no way it can get any worse than this.”

She was describing Aleppo in 2012.

“It got a lot worse,” she said. “Much worse.”

Last week, Syrian regime forces systematically executed civilians left in Aleppo, including children. A brief pause in the fighting between rebels and government forces to allow innocents safe passage out of the city ended when regime-backed militants opened fire on the buses dispatched to fetch those refugees.

There are many complicated reasons that Aleppo has become a hell worse than hell itself. There’s also a simple one: Barack Obama allowed it to happen.

Indeed, his decisions encouraged it.

Obama came to office determined to reach a deal with Iran on nuclear weapons at any cost. Obama made clear to the Iranians we’d tolerate their misbehavior in the region, so long as they gave him a deal. He told the Russians that he’d look past their expansionism if they backed the Iran deal. He called for Bashar al-Assad to go, but he made clear to the Iranians that the United States was unwilling to do anything to push their client out.

He was willing to pay any price for his deal, so long as others would bear the burden. And they have. The costs in Syria alone are staggering: nearly 500,000 dead. Millions more displaced. A country in ruins.

There is a horrific irony here. Obama’s single-minded pursuit of one legacy-defining accomplishment, the Iran deal, ensures that his legacy will be forever blackened by the tragedy of Syria. His presidency will be remembered as a time when America brushed aside its responsibility as a leader, ignored its responsibility to our fellow human beings, and turned a blind eye to the atrocities in Aleppo and elsewhere.

On this, at least, it will be remembered, in the words of one former proponent of American exceptionalism, as a betrayal of who we are.