In “Rise of the Warrior Cop,” Radley Balko chronicles the steady militarization of the police in the U.S. A detailed history of a dangerous trend, Mr. Balko’s book tracks police militarization over the past 50 years, a period that not coincidentally corresponds with the rise of SWAT teams. First established in response to the armed riots of the late 1960s, they were originally exclusive to big cities and deployed only against heavily armed and dangerous criminals. Today SWAT teams are nothing special. They’ve multiplied like mushrooms. Every city has a SWAT team; 80% of towns between 25,000 and 50,000 people do as well. These teams are busy; in 2005 there were between 50,000 and 60,000 SWAT raids in the U.S. The tactics are pretty much what you would expect—breaking down doors, rushing in with military weaponry, tear gas—but the targets aren’t. SWAT teams are routinely deployed against illegal poker games, businesses suspected of employing illegal immigrants and barbershops with unlicensed hair stylists.