Iraq and Iran are actually pretty distinct historically. Iran is the center of the old assyrian empire, a tradiional stronghold of Shiite islam. Iraq is the traditional center of sunni islam.

They pretty much form the front line of the roman / persian forever war, so they're probably some of the most constant antagonists in recorded history. Then there's this whole fault line in islam that runs between them. Then in modern history they had a really bloody war, that left a huge number of people seriously ill and disabled because of us-supplied Iraqi chemical weapon use.

I've been trying to think of a european equivalent and I've come up short because I simply can't think of any two polities that have been so consistently at odds, or so consistently under distinct and radically opposed rulerships. There's definitely more similarity between Japan and China, for example, or the USA and Russia.