“We have to do it right, not fast,” said the chairman, Army Gen. Martin E. Dempsey. “They have to have military leaders that bind them together. They have to … have a political structure into which they can hook, and therefore be responsive to. And that’s gonna take some time.”

The training is expected to be a centerpiece of the expanded U.S.-led effort to fight the Islamic State in Syria. U.S. officials plan to initially train 5,000 rebels in Syria, providing a ground force that can be paired with the airstrike campaign that the United States and partner nations launched there this week. But the effort could eventually expand beyond that, Dempsey said.

“Five thousand’s never been the end state,” the general said of the number of trained rebels he believes are needed in Syria. An estimated 12,000 to 15,000 would be needed “to recapture lost territory in eastern Syria.”