Arguably the trend starts amid the national political turmoil of the late 1960s, when police first started organizing heavily armed SWAT teams along military lines.
After Charles Whitman climbed to the top of the University of Texas tower with a rifle and fired on the people below, and local police in Austin had to deputize ordinary citizens to help stop him, departments began to fear the possibility of being outgunned by mass shooters or suspects who took hostages.
It was the Los Angeles Police Department that coined the term "SWAT" in 1967, originally called "Special Weapons Assault Team" until the then-chief decided that name was too militaristic. But there was always a political and racial component to these tactics. Even before the LAPD formed its team, SWAT-esque groups were deployed in response to protests by Cesar Chavez's United Farm Workers in Delano, California.
Not surprisingly, this trend escalated with the inception of Reagan's War on Drugs. In 1981, Congress passed the Military Cooperation with Law Enforcement Act, which "allowed and encouraged the military to give local, state, and federal police access to military bases, research, and equipment," as the Huffington Post reflected on the 10th anniversary of the Sept. 11 terrorist attacks.