What is now Iraq was largely almost always under Sassanid/Persian rule. The old Persian capital of Ctesiphon was basically next door to modern Baghdad. The old

What is now Iraq was largely almost always under Sassanid/Persian rule. The old Persian capital of Ctesiphon was basically next door to modern Baghdad. The old fault line was really in eastern Anatolia. Rome held Mesopotamia for a very brief period of time and didn't have much impact other than the occasional army passing through until the Muslim conquest.