"The Obama Administration identifies Iran as a major threat to U.S. national security interests. This perception is generated by suspicions of Iran's intentions for its nuclear program- heightened by a November 8, 2011, International Atomic Energy Agency (IAEA) report-as well as by Iran's support for militant groups in the Middle East and in Iraq and Afghanistan. Tensions have been particularly elevated since Iran's late December 2011 threat to try to choke off much of the world's oil supplies by attempting to close the Strait of Hormuz-a reaction to the imposition of significant sanctions against Iran's vital exports of oil. U.S. officials also accuse Iran of helping Syria's leadership try to defeat a growing popular opposition movement, and of taking advantage of Shiite majority unrest against the Sunni-led, pro-U.S. government of Bahrain. The sense of imminent crisis with Iran which greeted the beginning of 2012 follows three years in which the Obama Administration first offered Iran's leaders consistent and sustained engagement in exchange for limits to its nuclear program but, since 2010, has emphasized pressuring Iran through economic sanctions. Significant additional sanctions were imposed on Iran by the U.N. Security Council (Resolution 1929), as well as related 'national measures' by the European Union, Japan, South Korea, and other countries. Further measures intended to compel foreign firms to exit the Iranian market were contained in U.S. legislation passed in June 2010 (the Comprehensive Iran Sanctions, Accountability, and Divestment Act, P.L. 111-195). In late 2011, the Administration, Congress, and U.S. partners increased sanctions significantly by attempting to cut off transactions with Iran's Central Bank and through the European Union's January 23, 2012, imposition of an embargo on purchases of Iranian oil. The Administration also maintains a 40,000-50,000 troop military presence in the Persian Gulf, and it has stepped up arms sales to regional states that share the U.S. suspicions of Iran's intentions."
CRS Report for Congress, RL32048