During World War I, British forces invaded Mesopotamia in 1917 and occupied Baghdad. Before they succeeded, they suffered a major defeat at the hands of the Turkish army, the siege and surrender of Kut. At the end of the war, the Ottoman empire collapsed and an armistice was signed with Turkey in 1918.

Iraq was carved out of the old Ottoman Empire by direction of the UK government on January 10, 1919, and on November 11, 1920 it became a League of Nations mandate under British control with the name "State of Iraq".

At the end of the war, ownership of and access to Iraq’s petroleum was split five ways: 23.75% each to the UK, France, The Netherlands and the USA, with the remaining 5% going to a private oil corporation headed by Calouste Gulbenkian. The Iraqi government got none of the nation’s oil. This remained the situation until the revolution of 1958.

The British government laid out the institutional framework for Iraqi government and politics; the Iraqi political system suffered from a severe legitimacy crisis; Britain imposed a Hashemite monarchy, defined the territorial limits of Iraq with little correspondence to natural frontiers or traditional tribal and ethnic settlements, and influenced the writing of a constitution and the structure of parliament. Britain also had to put down a major revolt ( also known as the Arab revolt) against foreign rule between 1920 and 1922, resorting to aerial bombardment of Iraqi villages before control was established. These operations, in which it is alleged poison gas was used, were led by the future prime minister W.Churchill.