"According to the Administration's 'National Security Strategy' document released on March 16, 2006, the United States 'may face no greater challenge from a single country than Iran.' That perception intensified following the military confrontation between Iranian-armed and assisted Lebanese Hezbollah and Israel in July-August 2006. To date, the Bush Administration has pursued several avenues to attempt to contain the potential threat posed by Iran, including supporting a long term policy of changing Iran's regime. However, the Administration focus on preventing an Iranian nuclear weapons breakthrough has brought diplomatic strategy to the forefront of U.S. policy...Iran's nuclear program is not the only major U.S. concern on Iran. Successive administrations have pointed to the threat posed by Iran's policy in the Near East region, particularly material support to groups that use violence to prevent or complicate Israeli-Arab peace. Such groups have long included Lebanese Hezbollah and the Palestinian groups Hamas and Palestinian Islamic Jihad. Although there is no evidence of an operational relationship with Al Qaeda, some senior Al Qaeda activists are believed to be in Iran, although Iran claims they are 'in custody.' U.S. officials also accuse Iran of attempting to exert influence in Iraq by providing arms and other material assistance to Shiite Islamist militias, some of which are participating in escalating sectarian violence against Iraq's Sunnis there."
CRS Report for Congress, RL32048