Rebel fighters tear down a poster bearing the portrait of Syrian President Bashar al-Assad (right) and his late father and predecessor Hafez al-Assad (left)in Syria's eastern town of Deir Ezzor, on August 10, 2013. Photograph by Abo Shuja/AFP via Getty Images
Rebel fighters tear down a poster bearing the portrait of Syrian President Bashar al-Assad (right) and his late father and predecessor Hafez al-Assad (left)in Syria's eastern town of Deir Ezzor, on August 10, 2013. Photograph by Abo Shuja/AFP via Getty Images

The Central Intelligence Agency's number two, Michael Morrell, ranked Syria -- not Iran's nuclear program, al-Qaeda or China -- as the top current threat to U.S. national security in an interview with the Wall Street Journal as he retired from office on Friday. What he didn't say was that President Barack Obama has failed to deal with it.

Morrell is right to rank Syria so high, even though other security threats are bigger long-term issues for the U.S. to confront. The breaking of Syria into a failed state, and its concomitant sucking-in and spewing-back out of radical jihadis from around the region, is happening fast. This is opening new, potentially bad situations by the month.

Morrell's assessment came without comment on U.S. policy, but it supports my own view that Obama's handling of the conflict in Syria will be seen as his largest foreign policy failure, made in part as an overreaction to his predecessor's monumental error in invading Iraq.

That isn't to say that Senator John McCain was right in wanting a Libya-style U.S. intervention in Syria from the get-go in 2011, back when President Bashar al-Assad's troops were shooting unarmed pro-democracy protesters in the streets. Syria's civil war wasn't, and isn't, a conflict that the U.S. should try to own. But the Obama administration's resistance to doing anything at all to influence events on the ground inside Syria is likely to come at a future cost.

The administration has been reluctant to get involved in any of the events that have unfolded with the turmoil of the Arab Spring uprisings (even in the Libyan case, the U.S. was bounced into action by France). So Syria isn't the exception, it is the rule. This is an understandable response to the massive overreach committed by the administration of George W. Bush in Iraq in 2003. It is also fully supported by most Americans, who have zero interest in any further Middle East entanglements. Still, the U.S. can't pretend to be Belgium: Like it or not, it has big assets and commitments in the Middle East.

There were never any good options for the U.S. in Syria, but Obama should have gone with the advice of his chiefs at the Pentagon, the CIA and Department of State last year, when they recommended arming the Free Syrian Army in an effort to build up those factions on the battlefield that were most compatible with U.S. goals.

Opponents of arming Syria's rebels like to refer to the Afghanistan precedent, when the U.S. armed the Mujaheddin in their fight against the Soviet military in the 1980s, only to see some of those people and their weapons turn against the U.S., in the form of al-Qaeda. True, but here is another use of the Afghanistan comparison: Al-Qaeda was able to train, develop and organize, because the U.S. simply walked away from Afghanistan after the Soviets were driven out, leaving it to be torn apart by a civil war fueled by neighbors. The country became a failed state and a haven for Islamist radicals, who used it as a launch pad for attacks on the U.S. Eventually, the U.S. was forced to invade, at enormous cost.

This is the risk in Syria, a country in a neighborhood far more important to U.S. interests than Afghanistan, given Syria's borders with Iraq, Israel, Jordan, Lebanon and Turkey. Syria also has a greater potential for the misdirection of weapons, both conventional and chemical. Those weapons don't have to come from the U.S.; Assad has plenty of Russian ones to distribute or lose control of, while the radical Islamists have their own sources.

By doing nothing when advised to take some risk by arming and building up the Free Syrian Army, the administration left a vacuum that al-Qaeda and its affiliates have willingly filled. The recent U.S. decision to provide small arms is probably too little and too late to have any impact. The window to shape events without having to commit large numbers of troops has probably closed. No one can know whether earlier U.S. action would have made the difference, but U.S. policy has lost the allies it might have had in Syria and, I suspect, opened the door to worse outcomes and a much larger possible U.S. military intervention down the road.

(Marc Champion is a Bloomberg View editorial board member. Follow him on Twitter.)