The Pinkerton National Detective Agency was a private U.S. security guard and detective agency established by Allan Pinkerton in 1850. Pinkerton had become famous when he foiled a plot to assassinate President-Elect Abraham Lincoln. Pinkerton’s agents performed services ranging from security guards to private military contracting work. During its height, the Pinkerton Detective Agency employed more agents than the standing army of the United States of America, causing the state of Ohio to outlaw the agency due to fears it could be hired out as a private army or militia.

During the labor unrest of the late 19th century, businessmen hired Pinkerton guards to keep strikers and suspected unionists out of factories. The most notorious example was the Homestead Strike of 1892, when Pinkerton agents killed several people while enforcing the strikebreaking measures of Henry Clay Frick, acting on behalf of Andrew Carnegie, who was abroad. The agency’s logo, an eye embellished with the words “We Never Sleep” inspired the term “private eye.” The “Pinkertons” were also used as guards in coal, iron and lumber disputes in Illinois, Michigan, New York and Pennsylvania, as well as the railroad strikes of 1877.