The gap between our rhetorical policy and our actions constantly bedeviled U.S. policy making. In August 2011, several months after the start of the Syrian uprising and in the midst of the Libya operation, Obama joined our key European partners in declaring that “the time has come for President Assad to step aside.” But having learned the lessons of regime change in Iraq, and sobered by the complexity of sustaining even an air campaign in Libya, no principal argued for direct military intervention with U.S. ground troops to force out Assad, as President George W. Bush had done to Saddam Hussein. The costs in blood and treasure to the U.S. were massive in Iraq. Syria would be just as bad, if not worse, given Iran’s strong backing. Once Russia put its forces into Syria in September 2015, any effort at regime change could have courted World War III.

But we did consider and reconsider, again and again, many significant steps short of direct war against Assad. We imposed what U.S. and European sanctions we could, but absent UN Security Council authority, which Russia consistently blocked, comprehensive global sanctions were not achievable. We provided almost $6 billion in humanitarian assistance to the victims of Syria’s conflict and more to the neighboring states coping with the burden. We spent untold amounts of senior-level energy trying to negotiate with Russia, Syria, and other key players to end the conflict peacefully. At various points, we tried to exploit potential diplomatic openings, but none ever came to fruition.

After more than a year of intense internal debate, Obama decided in 2013 to join our Sunni Arab and Turkish partners in vetting, arming, and later training Syrian rebels. The challenge we continually faced, however, was that some of the rebels were genuine political opponents of Assad, while others were members of lethal terrorist groups. Still others were somewhere in between. The terrorist groups, such as the al-Qaeda-linked al-Nusra Front, were the best anti-Assad fighters, but we would never assist them. The difficulty was how to help the good guys, and those in the gray area, without inadvertently providing sophisticated weapons and training to terrorists.

We tried to walk that fine line. But the rebels were fractured and lacked a coherent, achievable political agenda. The assistance we provided was significant, but not as much as the rebels wanted and arguably needed. We did not do the maximum, because we assessed that the long-term risks of passing the most dangerous weapons to rebels in a murky war zone outweighed the benefits. While the U.S.-supported rebels fought as best they could, they were unlikely ever to threaten the regime’s survival without direct U.S. military intervention.

Within the Obama national-security team, the principals fought over Syria longer and harder than on any issue during my tenure. John Kerry, John Brennan, and Samantha Power argued for the U.S. to do more—provide more lethal weapons to the rebels, take targeted strikes against Assad or his air force, and perhaps establish safe zones for civilians. Others—including me and Denis McDonough, Defense Secretary Ash Carter, and Joint Chiefs of Staff Chairman Martin Dempsey and his successor, Joe Dunford—were equally tortured by the suffering in Syria, but opposed deeper U.S. military involvement.