What will Iran do?

If we attack Iran why wouldn't the Iranians attack our troops in Iraq with their troops? So far most of the attacks on our troops have come from the Sunnis who are opposed by Iran. Why wouldn't the Shia, of Iraq, then also begin to attack the American troops? The Shia have mostly been killing Sunnis, so far, but if we attack Iran, this would most certainly change.

Here is a good link and video.