August Vollmer was the first police chief of Berkeley, California, and a leading figure in the development of the field of criminal justice in the United States in the early 20th century. He has been described as "the father of modern policing". Vollmer played an influential role in introducing early 20th-century police reforms, which increasingly militarized police departments in the United States. A veteran of the Spanish–American War in the Philippines and the Philippine–American War, Vollmer introduced reforms that reflected his experiences in the U.S. military.

The militarization of police is the use of military equipment and tactics by law enforcement officers. This includes the use of armored personnel carriers (APCs), assault rifles, submachine guns, flashbang grenades, sniper rifles, and SWAT teams. The militarization of law enforcement is also associated with intelligence agency–style information gathering aimed at the public and political activists and with a more aggressive style of law enforcement. Criminal justice professor Peter Kraska has defined militarization of police as "the process whereby civilian police increasingly draw from, and pattern themselves around, the tenets of militarism and the military model".