2/ Control of Iran and Iraq gave the Sasanians (and other Persian dynasties going back to the Achaemenids) distinct advantages. With the Tigris and Euphrates, Iraq was an agricultural powerhouse. This also made it a tax collector's paradise 
3/ By contrast, the Iranian plateau was a great reservoir of manpower, with which an empire could fill an army.
Put simply, Iraq supplied the financial muscle of the state, while Iran supplied the military muscle
Put simply, Iraq supplied the financial muscle of the state, while Iran supplied the military muscle
4/ The Muslims conquered the Sasanian Empire in the mid-7th c. Suddenly Iraq and Iran were both under Arab control. But Iraq was conquered first, and thus it was from Iraq that the conquest of Iran was organized and launched
5/ After the conquest was over, large swathes of Iran continued to be controlled by Iraq. The two most important cities were the garrison towns of Basra and Kufa, both located in the south, which were home to large Muslim armies (below, Kufa today)
6/ But Iran would reassert itself over Iraq in due course. The ʿAbbasid Revolution (747-750) was hatched in far eastern Iran, in the region known as Khurasan. The Arab and Persian footsoldiers of the revolution violently toppled the Umayyad caliphs based in Syria
7/ A new ʿAbbasid caliphate was proclaimed, and in 762, it established its new capital at Baghdad, just up the river from the old Sasanian capital of Ctesiphon (below, one of the last portions of the ancient city which still stands, the monumental arch of Īwān Kisrā)
8/ In a way, we might see the ʿAbbasids as having restored the old Sasanian arrangement, combining the agriculturally productive farmlands of Iraq with the militarily muscle of Iran. It's no wonder that the ʿAbbasids saw themselves as heirs of the Sasanian kings
9/ Starting in the mid-9th c., the Iraqi heartlands of the ʿAbbasid caliphate went into decline. In the long term, this provided an opening for new powers to sweep in from Iran and control Iraq anew
10/ These included the Buyids, Shīʿīs from Daylam (a mountainous region in northern Iran), who seized Baghdad in 945; and the Seljuks, Sunnī Turks based in Khurasan (eastern Iran), who took Baghdad in 1055
11/ To sum up, Iran and Iraq have always been mixed up in each others' business. Yet the momentum of power and influence has switched from one side to another throughout history. We are living through one such shift in regional politics today
12/ If you're interested in geography and politics in the medieval Middle East, see this excellent essay by my teacher Michael Cook, "The Long-Term Geopolitics of the Pre-Modern Middle East," Journal of the Royal Asiatic Society 26 (2016): cambridge.org/core/journals/…
