Senior intelligence sources in the U.S., as well as officials in the Middle East, claim that the government of the Islamic Republic of Iran has made a strategic decision to confront American forces in Iraq's Shi'a heartland. Those senior intelligence sources (a total of five separate individuals who either now serve or have served in key intelligence positions) base their belief on evidence showing that Iran has armed Shi'a groups in southern Iraq with sophisticated weaponry, has provided political and military guidance to Shi'a groups, has made and maintained contacts with Sunni resistance leaders in "the Sunni triangle" in central Iraq, and is pursuing a program of escalating confrontations between Shia militias and American troops. Among the weapons shipped to the Shi'a militants are sophisticated anti-tank rockets and anti-aircraft missiles, according to these sources.

http://www.thisisrumorcontrol.org/node/view/287

"The rhetoric coming out of the Bush administration has convinced Iran that military conflict is inevitable and rather than await an attack at a time and place of America's choosing, the Iranians will try to inflict significant damage to U.S. forces on Iraqi soil by means of the Mahdi Army and other Shi'a groups," an informed intelligence source told This Is Rumor Control. Senior officials of the U.S. Central Intelligence Agency would not comment on these reports, but a former senior intelligence officer said that the conclusion was "a no brainer." As he noted: "If you had U.S. troops on your doorstep and George Bush calling you a part of the axis of evil you would take steps to protect yourself. And it would be better to protect yourself on Iraqi soil than to have to do so on Iranian soil. That is what they are doing. Are we surprised? We shouldn't be."