Before reforms (1534–1704), Iraq was divided into four eyalets (provinces):
Ottoman Iraq was later (1831–1920) divided into the three vilayets (provinces):
During World War I, an invasion of the region was undertaken by British Empire forces and was known as the Mesopotamian campaign. Fighting commenced with the Battle of Basra in 1914 and continued for the duration of the war. The most notable action was the Siege of Kut, which resulted in the surrender of the British and British Indian Army garrison of the town in April 1916, after a siege of 147 days. Of the 11,800 Allied soldiers who survived to be made prisoners, 4,250 died of disease or at the hands of their Ottoman guards during captivity.