Mr. Morell’s stark assessment shows how much the U.S. has at stake as it reluctantly prepares to arm Syrian rebels in the coming weeks while continuing to confront an al Qaeda that has dispersed across the globe. His forecast is all the more worrisome because it comes from a top official who other officials say is skeptical of current administration plans to arm the rebels…

“It’s probably the most important issue in the world today,” he said, “because of where it is currently heading.” Its current track is toward the collapse of its central government, he said.

He said there are now more foreign fighters flowing into Syria each month to take up arms with al Qaeda-affiliated groups, than there were going to Iraq to fight with al Qaeda at the height of the war there.

The Syrian government’s weapons, Mr. Morell said, “are going to be up for grabs and up for sale” as they were in Libya. The violence in Syria has the potential to spill over into Lebanon, Jordan and Iraq.