The mullahs are aware that a strong and unified Iraq is the only thing that stands between Iran and the first leg of the empire of Darius and Cyrus.
The ancient capital of the Persian Empire, Ctseiphon, is not far from Baghdad. Beyond that, the major shrines of Shiite Islam are in Iraq southwest of Baghdad. It may well be that the mullahs see themselves as the new military geniuses of a resurgent Persian Empire.
There is no way that we cannot understand that Iran can keep Iraq destabilized long after the United States pulls out. Iran is insuring that we know that.
Syria may well be behind the Saddam channel. If Syria were removed from the picture the channel would be reestablished in the Bekaa Valley in Lebanon. Hezbollah would run it. What would be different is that Lebanon has no contiguous border with Iraq. That would cut down a lot on the smuggling of people and explosives into Iraq.
Beyond that it is a direct challenge to the United States. The mullahs are well aware that as long as they can close the Straits of Hormuz the United States, and therefore the rest of the World, will not act to strike them. Israel is the only country that can perform the deep strike mission required to do any serious damage to their nuclear facilities other than the United States.
As it stands now, that attack must come through Saudi airspace. Once the first airplane crosses into Iranian airspace, the regional war is on. It appears to be the theory now that it is best to postpone this conflict as long as possible. it has always been my theory that if you know to pretty much a metaphysical certainty that there will be war, it is perhaps better to fight the war when you are ready and your opponent isn’t.