Answer
The Qazvin Cossack Brigade, led by Reza Shah in 1921, played a pivotal role in shaping Iran's modern history. This elite military unit, stationed in the city of Qazvin, northwest of Tehran, served as a catalyst for Reza Shah's rise to power. Under his leadership, the brigade successfully suppressed revolts and consolidated control over various regions, eventually leading to the establishment of the Pahlavi dynasty in 1925.