WASHINGTON - Iran, a country that has bedeviled the United States for decades, could prove to be the biggest foreign policy challenge facing whoever is the next president. The messy Iraq war and a spy scandal linking Pentagon and Israeli officials could complicate U.S. hopes of halting Iran's nuclear ambitions. Both President Bush and Democratic nominee John Kerry say they want to use diplomacy — although with different approaches — to prevent what could be a nightmare scenario for the United States: a nuclear-armed, hostile Islamic state in the volatile Middle East. But the United States' ability to sound an...