 Complete the following phrase:  The dynasty replaced the Qajar dynasty in the early 1920s, beginning on 14 January 1921 when 41-year-old soldier Reza Khan was promoted by British General Edmund Ironside to lead the British-run Persian Cossack Brigade. About a month later, under British direction, Reza Khans 3,000-4,000 strong detachment of the Cossack Brigade reached Tehran in what became known as the 