From Wikipedia:
Initially there were hopes that post-occupation Iran could become a constitutional monarchy. The new, young Shah Mohammad Reza Shah Pahlavi initially took a very hands-off role in government, and allowed parliament to hold a lot of power. Some elections were held in the first shaky years, although they remained mired in corruption. Parliament became chronically unstable, and from the 1947 to 1951 period Iran saw the rise and fall of six different prime ministers. Pahlavi increased his political power by convening the Iran Constituent Assembly, 1949, which finally formed the Senate of Iran—a legislative upper house allowed for in the 1906 constitution but never brought into being. The new senators were largely supportive of Pahlavi, as he had intended.
In 1951 Prime Minister Mohammed Mosaddeq received the vote required from the parliament to nationalize the British-owned oil industry, in a situation known as the Abadan Crisis. Despite British pressure, including an economic blockade, the nationalization continued. Mosaddeq was briefly removed from power in 1952 but was quickly re-appointed by the shah, due to a popular uprising in support of the premier and he, in turn, forced the Shah into a brief exile in August 1953 after a failed military coup by Imperial Guard Colonel Nematollah Nassiri.
Thanks. I should have looked it up.