BEIRUT, Lebanon — President Obama’s determination to train Syrian rebels to serve as ground troops against the Islamic State in Iraq and Syria leaves the United States dependent on a diverse group riven by infighting, with no shared leadership and with hard-line Islamists as its most effective fighters.

After more than three years of civil war, there are hundreds of militias fighting President Bashar al-Assad — and one another. Among them, even the more secular forces have turned to Islamists for support and weapons over the years, and the remaining moderate rebels often fight alongside extremists like the Nusra Front, Al Qaeda’s affiliate in Syria.

“You are not going to find this neat, clean, secular rebel group that respects human rights and that is waiting and ready because they don’t exist,” said Aron Lund, a Syria analyst who edits the Syria in Crisis blog for the Carnegie Endowment for International Peace. “It is a very dirty war and you have to deal with what is on offer.”

Analysts who track the rebel movement say that the concept of the Free Syrian Army as a unified force with an effective command structure is a myth.