While much about the new program remains unclear, the work of the command here since it began operations this year gives a sense both of how the United States seeks to build this force and the challenges it will face in doing so.

So far, the program has focused on a small number of vetted rebel groups from the hundreds that are fighting across Syria, providing them with military and financial help, according to rebel commanders who have received support.

The process is run by intelligence officials from a number of countries. The United States provides overall guidance, while Turkey manages the border, and Persian Gulf states like Saudi Arabia provide much of the funding.

Throughout Syria’s civil war, analysts have blamed the multitude of funding streams for creating a divided rebel movement with hundreds of groups seeking to please foreign backers.

This has changed in recent months. Turkey, which once allowed smugglers and fighters to move freely across its border with Syria, has clamped down, making it harder for private funders to get in.

At the same time, most of the support from governments who back the rebels is now channeled through the Military Operations Command.

This sidesteps the Syrian National Coalition, the exile body that is supposed to guide the rebellion but has little credibility inside Syria. Also sidelined was the coalition’s Supreme Military Council, which was widely accused of mismanagement and all but collapsed this year amid a leadership dispute.