When the Ottomans declare themselves formally allied to the German side, the British were quick to protect their interests and invade what is now known as Iraq. British and British Indian forces land at Al Faw, and meeting only minimal resistance they occupy Basra on 14 November 1914. By May the following year the British are well established in the south.

Initially the Ottomans put up a weak fight and General Townsend's army advances towards Bahgdad. At Ctesiphon the British engage regular troops of the Ottoman army and are forced to retreat to Al Kut where they are besieged for 140 days. In April 1916, with no supplies reaching them from the south, the British unconditionally surrender.

The following year under General Maude, the British launch a new campaign and by March 1917 Baghdad is captured. Advancing north they take Mosul in December 1918, giving them the three Ottoman provinces (vilayets) that will make up the future state of Iraq.

In June 1918, in a statement to Arab leaders in Iraq, Syria and Palestine, the British announce they intend to promote Arab independance. This was met with jubilation by all Arabs, but was a barefaced lie as the British and French had already divided up the Middle East in the secret Sykes-Picot Agreement of 1916. France to get greater Syria, England to get Iraq and Transjordan. Palestine was to be under international control, but France ceded all rights to England.

The territory that was to become Iraq was to be made from three distinct and separate Ottoman provinces with many diverse ethnic and tribal differences; predominantly Shi'ite Muslim Basra in the south, Baghdad in the middle and largely Sunni Muslim Mosul in the north. In the north and central west are a large minority of Kurds. This forced union of such diverse provinces would prove difficult to hold together.