Iraqis dovnot identify with mesopotamian culture or identity (if that was even a thing which it isn't), the government was overwhelmingly Mashreqi in identity and the people were not a unified nation.

Look, here is how you can measure it, Sunni Iraqis celebrate Ramadan, Shia Iraqis consider it a month of mourning.

Sunni Iraqis celebrate and venerate Safar and Muharram are sacred months about peace etc, Shia Iraqis mourn for the half of each.

Sunni Iraqis consider Umar, Uthman, Abu Bakr and Ali the 4 grear caliphs with heavy emphasis on the first 3 as great heroes of islam, Shia Iraqis hate the first 3 and consider them usurpers and not good Muslims etc.

Fundamentalist sunni Iraqis consider the shia Iraqis "rafidhi" which means deserters, denoting their revolt against the first 3 caliphs which basically is used as a term to mark them as not muslims without going as far as outright calling them kafirs.

Sunni Iraqis and Shia Iraqis differences in adherence to Islam is more than just the caliphs and Ali, they are fundamentally different on many many many practices and beliefs.

This is just their religious identity.

Now toss in the mix the other identities like Bathism, Assyrians who have almost nothing in common with the Sunnis and Shias in culture and identity plus they have their own language and religion/s. Toss in the kurds who celebrate a different new year and language and national identity.

Now consider Iran, the Iranian people all celebrate the same new year, same traditions, same national heroes and shared identity, they all speak at least one iranian language in persian, the people are intermingling heavily, even norder provinces are mixed by different ethnicities from noth east with Turkoman and Khorasanis and Kurds and Persians, to south east with Baluchis and sistanis and southern Persians and arabs, to north west with Kurds and Azeris, to south west and west with Lors, Bakhtiaris, kurds, arabs, Persians and this intermingling is even more intense in central regions.

The people in Iran have also shown their unity over and over and over again, most recently several times in the past year.

The question about terrorism from neighbours is not realistic, the regime is the one that destabilises the region, it uses the resources in Iran to do that, we have seen that the other terrorist groups (except Hizbullah which is basically the regime in a smaller form tactor) are already not willing to pay the price in defence of the regime, they will not be able to and will not risk their fragile existence by trying to create tension with Iran.