At the age of 16 he joined the Cossack Brigade, and also served in the army. In 1911 he was promoted to First Lieutenant, by 1912 he was elevated to the rank of Captain and by 1915 he became a Colonel. In February 1921, as leader of the entire Cossack Brigade based in Qazvin he marched towards Tehran and seized the capital. He forced the dissolution of the government and installed Zia al-Din Tabatabaie as the new Prime Minister. Reza Khan’s first role in the new government was Commander-in-Chief of the Army and the Minister of War. After the deposition of Ahmad Shah by the Parliament on October 31, 1925, Reza Khan was declared the new King of Persia by the Assembly and was given the title of Reza Shah Pahlavi.