The South Persia Rifles was a military organization established by the British government in Iran in 1916, during World War I. This research examines the formation of this military force known as the South Persia Rifles in Iran, its colonial objectives as pursued by the British government, and its functions until its dissolution. The physical presence of these British forces from the very first days of their formation faced widespread opposition from the Iranian people and government, particularly as the failure of the Iranian government to recognize this illegal British action was seen as interference in Iran's internal affairs. The fact that it was not approved by the Iranian Majlis (parliament) indicated that the South Persia Rifles was an illegitimate organization in Iran, and no matter how hard the British tried, they could not impose its recognition and legalization on the Iranian government. Eventually, the activities of the South Persia Rifles in Iran ended after five years in 1921, and it was dismantled. The present research is descriptive-analytical, and the tools for collecting data and information are derived from library sources and documents on Iran's foreign relations with Britain. The research findings show that with the formation of this British military organization in Iran, the British government, under the command of General Sir Percy Sykes, took the first strategic step towards colonizing Iran. However, it faced strong opposition from the Iranian government and fierce resistance from the Iranian people, especially the clergy, tribesmen, and villagers of southern Iran. If it had not been for this national and religious opposition of the Iranian people and the relative cooperation of the political authority of the time, perhaps Iran would have had a fate similar to that of India and other British colonies.