U.S. and Western military officials are calling on Iran to help stabilize Afghanistan, but historic tensions and allegations of arming militants raise doubts about Tehran’s intent.

The U.S. government designates Iran as the "most active state sponsor of terrorism," which feeds concerns about Iran’s growing nuclear program.

While the American public focuses on bringing U.S. forces home from Iraq and Afghanistan, defense planners in Washington consider what to do with them when they get back.