“In my 36 years working national security issues it is the most complicated issue I ever have encountered,” he said at the Washington-based Center for Strategic and International Studies think tank.
Thwarting a quick fix to Syria’s woes are a myriad of competing internal and external actors, sectarian divisions and years of suppression under the Assad family that has suppressed internal democratic aspirations, he said.
“The Arab Spring sort of opened that up,” he said, pointing to social upheavals that spanned North Africa and the Levant beginning in late 2010.
When those protests hit Syria the following year, however, President Bashar al-Assad launched a violent crackdown on protests, sending the country into a vicious cycle of violence that continues to this day.
“A lot of people have complained about the inability of the United States to go out there and to resolve a lot of these issues,” Brennan said. “Well, I wish we had that magic wand. Despite the challenges that we still face there, good on the United States for trying.
"Good on the United States for continuing to try to reduce the humanitarian suffering and the bloodshed that is there, and recognizing that we don’t have the solutions that can be imposed and forced upon the people," he added.
The country is in the midst of a recently-enacted fragile nationwide cease-fire brokered between Washington and Moscow, which back the opposition and government respectively.
U.S. officials have said the relative calm the agreement has ushered in is a far cry from the violence seen in previous days and weeks.
Still, Brennan acknowledged it “is still going to take a number of years” to find a resolution to the bloody conflict.
"The Arab Spring ushered in a new phase in Middle Eastern history, but there is still a long way to go before democratic principles are going to take root there and the economic, political, social, cultural and other types of reforms are going to be able to address what I think are very, very serious challenges,” he said.