1921 Persian coup d'état, known in Iran as 3 Esfand 1299 coup d'état, refers to several major events in Qajar Persia in 1921, which eventually led to the deposition of the Qajar dynasty and the establishment of the Pahlavi Empire as the ruling house of the country in 1925.

The Persian Cossack Brigade, also known as the Iranian Cossack Brigade, was a Cossack-style cavalry unit formed in 1879 in Persia. It was modelled after the Caucasian Cossack regiments of the Imperial Russian Army. Until 1920, it was commanded by Russian officers, while its rank and file were composed of ethnic Caucasians and later on Persians as well. During much of the brigade's history it was the most functional and effective military unit of the Qajar dynasty. Acting on occasion as kingmakers, this force played a pivotal role in modern Iranian history during the Persian Constitutional Revolution, the rise of Reza Shah, and the foundation of the Pahlavi dynasty.