Saudi Arabia will be a big part of the Obama administration's push for an international coalition to fight the Islamic State extremist group. Iran is already in that fight, backing a new government in Baghdad and helping arm the Kurds.
So how does Washington balance long-standing mistrust between Tehran and Riyadh with Iran's informal cooperation against the Islamic State?
In the fight against Islamic State militants in Iraq, it is U.S. airstrikes that are backing advances by Kurdish forces.
But the president of Iraq's Kurdistan region, Massoud Barzani, says it is Tehran that is helping Kurds on the ground.
"We asked our friends to arm our forces and send us weapons, and the Islamic Republic of Iran was the first country that provided us with the required weapons," he said.