The British also used Iran to prevent Russia from driving south. One of the means Russia used to exert its power was the Iranian Cossack Brigade, a military group, founded by Nassir al-Din Shah after a visit to Russia. The Brigade had Russian officers. Reza Khan joined the Cossacks at the very young age of fifteen or sixteen. As the years progressed Reza Khan witnessed the constant struggle of Iran against foreign interference in Iranian affairs. The influence of Britain and Russia remained in Iran during the beginning of the 20th century. The struggle between the two countries to establish power and maintain control within Iran led Reza Khan to question the leadership ability of the current shah and the Qajar dynasty. Reza Khan felt that Iran was in need of a strong militaristic leader that would free Iran from foreign domination.
In 1920 a man by the name of Major General Sir Edmund Ironside relinquished control of the British troops in Iran. Ironside was struck by the personality and physique of Reza Khan, who was now a colonel in the Cossack army. Ironside told Reza Khan the British would not oppose his seizing power so long as the reigning Qajar shah was not actually deposed. Reza Khan with the support of the Cossack Brigade mounted a coup to impose a new government on the weak Qajar leadership. A successful coup lead to the weakening of the Qajar dynasty. Because of his leadership in the coup, Reza Khan held key government positions that held him on way to become Shah of Iran. Soon after the coup, Reza Khan became minister of war and defacto commander-in-chief of the armed forces. His leadership rank in the army allowed him the power to begin modernizing the army. In 1923 he became Prime Minister and began to strengthen the central government against the major tribes- the Bakhtiari, the Qashqai, the Kurds and the Lurs. He felt the tribes contributed nothing to the country.