With the Ottoman empire joining the central powers in World War 1, Iraq was occupied by British forces. After the war, the country became a British Mandate territory on behalf of the League of Nations. The British set up a constitutional monarchy, under king Faisal I (reigned 1921-33), with an elected parliament. Iraq became independent in 1932. Sadly, politics became increasingly faction-ridden, which led to an army coup in 1936. From then on the army dominated politics until World War 2.

The economy continued to expand; irrigated farmland was extended, oil revenues became an important source of revenue, and the railway network was completed. In World War 2, the country was again occupied by the British after a pro-Nazi coup threatened British positions in the Middle East. A series of weak, short-lived civilian governments held power, but still managed to preside over much economic progress. This was largely thanks to a non-political but powerful agency known as the Development Board, which oversaw economic development.

In 1958, the king, Faisal II, was assassinated in a coup led by al-Karim Qasim, and a republic was declared.