And they know we aren't going to invade them with the current troops levels in either Iraq or Afghanistan. So the argument that they are only taking defensive measures is lacking.
Being bordered by Iraq and Afghanistan, I don't think they know they aren't going to be invaded. In fact, with US troops positioned to their left and right, if I'm Iranian, I probably have concerns. If (pick any country) had taken up positions in Canada and Mexico, I don't think we'd be too comfortable.