This piece is part of a series on Obama’s legacy that The Huffington Post will be publishing over the next week.

WASHINGTON ― The bombs kept falling after they had destroyed all the hospitals.

They kept coming as international aid agencies warned about the thousands of Syrian children they could hit.

They didn’t pause as temperatures fell below freezing, threatening families with no shelter, or as pro-regime fighters came close enough to civilians to start killing them execution-style, in the streets and in their homes. An “evacuation” was in the offing, the government promised. Residents knew what that meant: abandoning their homes, risking arrest and torture and being shuttled to an unfamiliar region that the regime would eventually target even more brutally.

By Dec. 16, Syrian President Bashar Assad’s militiamen had detained hundreds of evacuees and thousands of people had already died. And President Barack Obama stood at his last White House press briefing and said he had made the right call in not using the world’s most powerful military to protect the tens of thousands of people in the city under attack, Aleppo and those affected by Syria’s civil war in general.

“It was going to be impossible to do this on the cheap,” the president said, after arguing that only a major U.S. ground presence could have prevented the ongoing war crimes.

But many of Obama’s diplomats, responsible for years of negotiations over the conflict, disagreed.

In a June letter sent through an internal dissent mechanism, 51 State Department officials said there was a way to spur diplomatic efforts to prevent the massacres in Aleppo and elsewhere. And it didn’t involve the kind of ground invasion or chest-thumping show of “credibility” that Obama repeatedly said was the only alternative to his inaction.

“None of us sees, or has seen, merit in a large-scale U.S. invasion of Syria,” the officials wrote. “But we do see merit in a more militarily assertive U.S. role in Syria, based on the judicious use of stand-off and air weapons, which would undergird and drive a more focused and hardnosed U.S.-led diplomatic process.”

Privately, Secretary of State John Kerry frequently urged a similar step.

After spending years on painful nuclear negotiations with Iran to reduce the risk of a U.S. conflict with the country, Kerry was hardly one of the warmongers who Obama associates with the idea of military action against Assad. He has also avoided the speculative talk of people who had wanted Obama to strike Assad in 2013 or spent years calling for stronger weapons to be sent to anti-regime forces that included growing numbers of extremists. All along, he believed diplomacy was key. But he felt it would never be effective without a signal that the U.S. would act if it didn’t ― that diplomacy only matters if it is linked to facts on the ground, and that those on the other side of the table, Assad and his friends in Moscow and Tehran, were creating facts rapidly while deliberately stalling diplomacy.

Obama’s response to that idea reflects the principle that guided his approach to the world beyond American shores: that the U.S. should constantly think about the limits of its power.

PHILIPPE DESMAZES via Getty Images Photographs documenting Assad's torture of Syrians have been displayed across the world. They are seen here in Geneva, Switzerland, last March.

The outgoing president has stuck to that notion through diplomatic victories achieved by toning down U.S. militarism and overreaction — notably progress on American relationships with Iran, Cuba and Myanmar, and the peaceful transfer of hundreds of detainees from the Guantanamo Bay detention facility to other countries. And he’s held to it through moments like the fall of Aleppo — reminders that his approach entails major instability for years ahead and major damage to America’s ability to speak of global human rights. As he exits the White House, there is little certainty that Obama is leaving behind a world safer for all or better for the U.S.

Obama’s approach bore fruit in situations where traditional American actions were counterproductive to U.S. goals. By easing the embargo on Cuba while remaining critical of its rulers’ repression, he helped weaken skepticism of the United States’ position on the island. That meant Washington had fresh goodwill in Latin America and a new opportunity to try to engage on issues like human rights, while both the U.S. and Cuba could benefit from previously impossible trade.

His landmark diplomacy with Iran was more fraught, given that it meant tacitly permitting Assad’s butchery. But it worked in a similar way: By trying financial sanctions and direct diplomacy rather than sabre-rattling, Obama secured an agreement that experts and Iran skeptics agreed was a powerful, historic check on Iran’s nuclear ambitions. He reduced the risk of a fresh war or nuclear arms race in a turbulent region and curbed nuclear proliferation in general. Along the way, he supported his team in establishing a rare channel with Tehran that enabled the release of three Americans held in Iranian jail.

Because Obama’s “America-must-do-less” approach required other countries’ help to work, it allowed the U.S. to reach important compromises by acknowledging others’ goals as valid and identifying where those nations could have mutual interests with Washington. This is how he successfully wooed China to become a top backer of the Paris climate deal, for instance, and developed tacit coordination with Iran to battle the vicious Islamic State group and support the shaky but essential central government in Iraq.

This has been part of his approach to Europe, where he has tried to address the specific needs of NATO partners — beefed-up border security with Russia, mostly — while prodding the European Union to do more for its own protection, like maintaining unity behind German Chancellor Angela Merkel in the face of their financial crisis and upholding painful EU sanctions on Russia to show that there are consequences for actions like its 2014 invasion of Ukraine.

Kevin Lamarque / Reuters German Chancellor Angela Merkel was President Barack Obama's favorite foreign partner, he indicated to The Atlantic last year.

But this emphasis on America’s limits entails major costs — most dramatically where Obama has cited others’ power as a reason to avoid American action.

Syria is the prime example. After he decided not to hit Assad in retribution for his chemical attack on more than 1,000 civilians in 2013, Obama often spoke about how he forged a deal with Moscow over the Syrian dictator’s chemical weapons stockpile and how Syrian opposition groups would never be capable of unseating the tyrant’s decades-old army. Once the rebels’ success against Assad so threatened his regime that Russia came to his aid in 2015, the president’s team began telling reporters that U.S. action was even more difficult because it might lead to World War III.

Each talking point tried to mask how this was a self-fulfilling prophecy. Seeing the situation as analogous to Iraq, despite the fact that it involved homegrown Syrian dissent rather than externally imposed regime change, Obama believed the U.S. could do little to alter the situation in Syria. That shaped the United States’ response — and ensured that there was, in fact, little America could eventually do.

Assad’s regime, the chief driver of the refugee crisis weakening Europe and of cynical policies that benefit groups like ISIS, will now outlast Obama’s and likely continue using chemical weapons. Meanwhile, the president must deal with the consequences of choosing not to release information about Russian hacking of the American presidential election to try and protect the pretence of U.S.-Russia negotiations over Syria.

Obama’s approach worsened other crises because he wedded his perception of reined-in American power to the idea that others should take the lead on international problems. In some instances, that meant allowing U.S. partners to act in ways that ended up fueling militancy and anti-Americanism.

By signaling that Saudi Arabia should act as it saw fit in Yemen and not expect Washington-made policy, Obama enabled hundreds of war crimes, the starvation of tens of thousands and the creation of a power vacuum that ISIS and Al Qaeda have exploited. In Libya, he lost the potential gains of a successful humanitarian intervention because he silently allowed the United Arab Emirates, Egypt, Qatar and Turkey to establish a proxy war, leaving the tussle to develop for years before trying to step in. And in Iraq, the Obama team’s eagerness to hand off a problem and fulfil his campaign promise of bringing home U.S. troops permitted a sectarian, brutal prime minister to mistreat his population, convincing thousands of Iraqis that ISIS was their only protector. In recent years, he has been silent as Turkey has become increasingly repressive ― in part because he is desperate to maintain some kind of dialogue with the NATO ally, since he worsened U.S.-Turkish ties by avoiding tougher action on Assad and focusing his Syria policy on Kurds linked to terrorism within Turkey.

The president’s response when people note his acquiescence to such excesses is that other nations will realize the error of their ways. Russia, he predicted, would feel the pain of being bogged down in Syria; Saudi Arabia and other U.S. partners would come to prioritize political reform and opportunities for their own repressed populations over fixating on Iran. With Russian President Vladimir Putin thrilled about Assad’s survival and the heavily armed Arab Gulf states even angrier with Iran, these lessons now seem more elusive than ever. Meanwhile, human misery and disillusionment with the United States’ soaring rhetoric is in growing abundance.

TIMOTHY A. CLARY via Getty Images Protestors with the group Avaaz in 2012.

Where he has been compelled to act in some form, Obama has stuck to the view that U.S. moves are unlikely to be decisive. That’s inspired band-aid style actions that seem designed to save face, like tough talk on Chinese expansionism without strong efforts to make America’s partners commit to international law rather than give in to Beijing.

The president’s policy of slowing down the withdrawal of American troops from Afghanistan fits the bill. Although it’s done little to permanently protect the fragile Afghan government from an increasingly assertive Taliban insurgency, it has helped prevent a complete collapse of the country on Obama’s watch.

So does his much-lauded policy of “surgical” precision in targeting militant fighters using drones and other means. It removes top-tier figures in the Islamic State and Al Qaeda ― providing flashy headlines ― without addressing the fundamental problems in the Muslim-majority world and elsewhere that have aided their rise. Obama has, meanwhile, sharply reduced U.S. government attempts to tackle those problems, through engagement with civil society and efforts at political reconciliation.

“Obama is now using force to defeat ISIS while abjuring the work necessary to build something with which to replace it. That path bodes ill for the anti-ISIS project he has launched, and recreates for the next U.S. president the same dysfunctions in U.S.-Arab relations—moral hazard, security overcommitments, and the like—that Obama resents,” Tamara Cofman Wittes, a former top State Department official and current senior fellow at the nonpartisan Brookings Institution, wrote earlier last year.

“It is a tragic irony: A president elected and reelected on a platform of ending wars in the Middle East has reproduced, at the end of his presidency, the very situation he inherited, decried, and swore to avoid: an escalating war against a vague terrorist enemy, with no geographic boundaries, no clear military or strategic objectives, and no principles or policies that might stop the slide down this slippery slope.”

Although she believes the president is right to target ISIS, Wittes explained, she worries that he and his supporters will continue to feel that the terrorist group’s rise is evidence that his reluctance to engage with the Middle East was justified — rather than a sign that inaction has heavy penalties.

President-elect Donald Trump has yet to lay out any kind of coherent diplomatic strategy. But in some important ways, the Obama years created an opening for what seems to be in store: An abandonment of the idea that moral priorities should guide foreign policy in favor of a deep conviction that others — other nations, other peoples — should deal with their own problems however they see fit.