Defense Secretary Donald H. Rumsfeld urged Iraqi political leaders today to settle their differences and agree on a new constitution quickly, and to exert more influence with Syria and Iran to force them to end support for the insurgency here. Speaking to reporters en route to an unannounced visit here, Mr. Rumsfeld laid out a remarkably blunt prescription for what Iraqi leaders must do in the coming weeks and months to ensure that a stable, secure and popularly elected government survives, and to allow American troops to begin to withdraw. Full Story