The Mesopotamian campaign, part of the Middle Eastern theatre in World War I, was a conflict between the Allies (mainly the British Empire with troops from Britain, Australia, and predominantly the British Raj) and the Central Powers, predominantly the Ottoman Empire.^[54] Initiated in 1914, the campaign aimed to protect Anglo-Persian oil fields in Khuzestan and the Shatt al-Arab, eventually escalating to a broader objective of capturing Baghdad and diverting Ottoman forces from other fronts. The campaign concluded with the Armistice of Mudros in 1918, leading to Iraq's cession and further partitioning of the Ottoman Empire.

The conflict commenced with an Anglo-Indian division's amphibious landing at Al-Faw, swiftly moving to secure Basra and the nearby British oil fields in Persia (now Iran). The Allies achieved several victories along the Tigris and Euphrates rivers, including defending Basra at the Battle of Shaiba against an Ottoman counter-offensive. However, the Allied advance was halted at Kut, south of Baghdad, in December 1916. The subsequent Siege of Kut ended disastrously for the Allies, leading to a devastating defeat.^[55]

After reorganizing, the Allies launched a new offensive to capture Baghdad. Despite strong Ottoman resistance, Baghdad fell in March 1917, followed by further Ottoman defeats until the Armistice at Mudros.

The end of World War I and the subsequent defeat of the Ottoman Empire in 1918 led to a radical reconfiguration of the Middle East. The Treaty of Sèvres in 1920 and the Treaty of Lausanne in 1923 dismantled the Ottoman Empire. In Iraq, this ushered in a period of British mandate, as per the decisions of the League of Nations. The mandate period saw the establishment of the modern state of Iraq, with its borders drawn by the British, encompassing diverse ethnic and religious groups. The British mandate faced challenges, notably the 1920 Iraqi revolt against British administration. This led to the 1921 Cairo Conference, where it was decided to establish a Hashemite kingdom under Faisal, heavily influenced by Britain, in the region.