From his speech to the American Legion earlier today:

The other strain of radicalism in the Middle East is Shia extremism, supported and embodied by the regime that sits in Tehran. Iran has long been a source of trouble in the region. It is the world’s leading state sponsor of terrorism. Iran backs Hezbollah who are trying to undermine the democratic government of Lebanon. Iran funds terrorist groups like Hamas and the Palestinian Islamic Jihad, which murder the innocent, and target Israel, and destabilize the Palestinian territories. Iran is sending arms to the Taliban in Afghanistan, which could be used to attack American and NATO troops. Iran has arrested visiting American scholars who have committed no crimes and pose no threat to their regime. And Iran’s active pursuit of technology that could lead to nuclear weapons threatens to put a region already known for instability and violence under the shadow of a nuclear holocaust.
Iran’s actions threaten the security of nations everywhere. And that is why the United States is rallying friends and allies around the world to isolate the regime, to impose economic sanctions. We will confront this danger before it is too late. (Applause.)
I want our fellow citizens to consider what would happen if these forces of radicalism and extremism are allowed to drive us out of the Middle East. The region would be dramatically transformed in a way that could imperil the civilized world. Extremists of all strains would be emboldened by the knowledge that they forced America to retreat. Terrorists could have more safe havens to conduct attacks on Americans and our friends and allies. Iran could conclude that we were weak — and could not stop them from gaining nuclear weapons. And once Iran had nuclear weapons, it would set off a nuclear arms race in the region.