Iran hoped to avoid entanglement in World War I by declaring its neutrality, but ended up as a battleground for Russian, Turkish, and British troops. When German agents tried to arouse the southern tribes against the British, Britain created an armed force, the South Persia Rifles, to protect its interests. ThenÍÍÍÍ a group of Iranian notables led by Nezam os Saltaneh Mafi, hoping to escape Anglo-Russian dominance and sympathetic to the German war effort, left Tehran, first for Qom and then for Kermanshah (renamed Bakhtaran after the fall of Mohammad Reza Shah in 1979), where they established a provisional government. The provisional government lasted for the duration of the war but failed to capture much support.

At the end of the war, because of Russia's preoccupation with its own revolution, Britain was the dominant influence in Tehran. The foreign secretary, Lord Curzon, proposed an agreement under which Britain would provide Iran with a loan and with advisers to the army and virtually every government department. The Iranian prime minister, Vosuq od-Dowleh, and two members of his cabinet who had received a large financial inducement from the British, supported the agreement. The Anglo-Persian Agreement of 1919 was widely viewed as establishing a British protectorate over Iran. However, it aroused considerable opposition, and the Majlis refused to approve it. The agreement was already dead when, in February 1921, Persian Cossacks Brigade officer Reza Khan, in collaboration with prominent journalist Sayyid Zia ad Din Tabatabai, marched into Tehran and seized power, inaugurating a new phase in Iran's modern history.

Data as of December 1987