Historically Iranians were ruled by an absolute monarchy for several thousand years, at least since the time of the Achaemenid Empire (550 B.C.E.) until the Constitutional Revolution in the early 20th century. The Constitutional Revolution in 1906 replaced the absolute monarchy with a constitutional monarchy. The constitution went under several revisions during the following decades. During World War II Iran stayed neutral but in 1941 the Allies (the USSR and Great Britain) invaded Iran and replaced Iran's Shah Reza Pahlavi (who was perceived as being pro-German) with his son Mohammad Reza Pahlavi to protect their access to Iranian oil, and to secure routes to ship western military aid to the Soviet Union. Iran's parliamentary government led by Prime Minister Mohammed Mosaddeq was toppled in a 1953 coup d'état by royalist forces supported and funded by CIA and MI6 after Mohammed Mosaddeq nationalized Iranian oil. Shah Mohammad Reza Pahlavi became the preeminent leader in Iran, and instated Fazlollah Zahedi from the military as the new Prime Minister. United States has considered the Shah as a close ally and Iran as its main base in the Middle East. The Shah also tried to modernize Iran's economy and westernize Iran's culture. These and other policies contributed to alienating nationalists, leftists, and religious groups.