Rules of Engagement currently enforced by U.S. troops in Iraq:
They cannot fire first. They can watch an insurgient load a mortar and launch it, or pull a gun and point at them directly, but are not allowed to fire to stop these actions until the mortar is launched or the bullet leaves the chamber from the opposers gun, considering he isn't dead by this time.
If in doubt I can give e-mails of persons currently in Iraq for confirmation upon request. pm me.