Iran’s nuclear program is believed to have steadily progressed, despite sharply increased concerns over its intentions and sanctions over its lack of transparency.

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 contentious border between Pakistan and Afghanistan remains a site of major conflict, and houses some of the world’s most dangerous militants.

Backgrounder: North Korea is the latest member of the world’s still-exclusive nuclear club. But other aspirants are waiting in the wings.