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.

I didn’t see a feasible middle ground. Ordering limited strikes to punish Assad for using chemical weapons was one thing; assuming a larger role in Syria’s civil war would be something else entirely. If we took action that directly targeted Assad or his military, we would be at war with him, Iran, and ultimately Russia. If we set up a no-fly zone or safe zones on the ground, we were buying a costly, dangerous, lengthy, and uncertain military commitment on top of Afghanistan and Iraq that put significant numbers of U.S. forces in harm’s way. Could we have protected civilians in safe zones? Yes, had we deployed thousands of U.S. troops to take and hold the ground and committed roughly 100 planes to provide air coverage. But would that have toppled Assad? That was unlikely even before Russia intervened in September 2015.