I've been wondering a lot about why the Iraqi boarders were drawn up the way they are. It seems to create friction from all three major Muslim sects within the same boundary. I've heard some say this was on purpose, that the British wanted the Kurds, Sunni, and Shia to fight so the British could maintain power for longer. Yet I've also read that it was strategic in that bringing the north in provided more oil. Also that boundaries were drawn in the way it was easiest for the British to defend while their military was weak after WW1.
Is there any evidence that points to one or the other? Or is it a mixed bag and all of these reasons played a part in the creation of Iraq?
Mesopotamia had been an Ottoman province for a very long time - since 1535. The Ottomans themselves made little difference between ethnic groups, instead preffering to organise society along religious lines.
The Ottomans captured Mesopotamia from the Shia Safavid (Persian) Empire in a long and costly struggle. As the Ottoman Empire was officially Sunni and rejected Shia beliefs, the Ottomans made the local Arab Sunni population the elite. This system was kept up until the age of nationalism.
When the British captured Mesopotamia and created the Kingdom of Iraq, they allowed the local Sunni elites to remain and rewarded their Hashemite allies with the throne of Iraq.
Administrating the country like that had worked well for the Ottomans for centuries, and while tragic, no-one at the time cared for the Kurds.
Peter Slugett notes in Britain in Iraq: Contriving King and Country that Britain's interest in Iraq grew initially out of its need to protect access to India, though there was interest also in protecting trade in the Gulf and with southern Iraq. Also in the early 1900s, interest in oilfields grew among western powers, and though no major oil discoveries were made in the Ottoman Empire before 1914, oil had been discovered in SW Persia. Leading up to WWI then, Britain had concerns about the Ottoman Empire's involvement on the side of the Central powers, namely that an alliance with Germany would threaten their trade and communication with India and their access to the Persian oil fields.
When Britain initially sent Indian Expedition Force 'D' into Basra, it was more about obstructing the Ottomans rather than an all-out conquest--Sluglett calls it a "holding expedition." Nevertheless, Britain's views on Iraq rapidly evolved, and Britain found itself in control over the 3 distinct regions that comprise Iraq (Mosul, Basra, Baghdad) in Nov 1918. In A History of the Modern Middle East, William Cleveland asserts that the British authorities were "most arbitrary" in the formation of Iraq.
The specifics of the type of control and administration that Iraq could fall under become sticky, nevertheless, post-WWI, outright annexation wasn't acceptable, so other, more subtle means of control and to be employed. Imposition of more centralized control particularly riled the independent tribes who were not connected to the city centers. Although not explicitly nationalist, a tribal uprising in 1920 is viewed as the first "symbol of the new state's rejection of foreign rule." Although a military occupation was viewed as necessary, the British sought to quickly establish an Arab government consisting of Sunnis who had supported Faysal during the Arab Revolt.