Mesopotamia (modern day Iraq) was a key area for Britain:
- At the head of the Persian Gulf, it was the gateway to British India and if the Germans and their Ottoman allies could cut this link it would be much harder for the massive Indian Army manpower to be made available;
- The British Navy was converting from coal to oil, so control of the developing oilfields in Persia (modern day Iran) and Mesopotamia was going to be critical;
- It was part of the Ottoman Empire.
Persia was neutral but was severely affected by the war, with the Allies controlling its natural resources – especially oil – and using its agriculture to feed the huge army in Mesopotamia.