In the wake of the United States’ war declaration against Germany on April 6, 1917, dozens of extralegal vigilance organizations such as the Knights of Liberty, American Rights League, Boy Spies of America, American Defense Society, Sedition Slammers, National Security League, and the Terrible Threateners sought to ensure Americans’ full participation in the war effort, often through measures of intimidation, harassment, surveillance, and outright violence. 

By far the largest of these hyper-patriotic organizations was the American Protective League, or A.P.L., which maintained a network of branches in more than 600 cities.  Like the aforementioned groups, the A.P.L. worked to enforce patriotism and stifle dissent.  Unlike these other bodies, however, the A.P.L.’s actions were carried out with the approval of the U.S. government. 

Indeed, with the quiet consent of the Department of Justice, the American Protective League’s 250,000 civilian members—many of whom wore official-looking badges reading “Secret Service”—undertook vigilante actions against supposedly disloyal socialists, pacifists, and immigrants; engaged in domestic surveillance operations; raided businesses, meeting halls, and private homes in an effort to uncover pro-German sympathizers; and bullied citizens who, it was believed, were less than fully committed to the country’s war endeavors.  The organization’s most notable action occurred in September 1918 when, along with local police and federal agents, thousands of A.P.L. operatives conducted a three-day “slacker raid” in New York City, resulting in the arrest and questioning of more than 75,000 suspected draft dodgers. 

The most thorough contemporary account of the A.P.L.’s wartime activities is Emerson Hough’s The Web: A Revelation of Patriotism, which was published in 1919.  Said The New York Times in its review of the book, “There are many reasons why every man and woman in the whole country ought to know the full story of the A.P.L.’s work.  For if they are good Americans already it will make them still better ones, will stimulate their pride and loyalty.  If they are not good Americans it will put the fear of God in their hearts.”

For a more recent history of the American Protective League, Joan Jensen's The Price of Vigilance (1968) is worth investigating, as is Bill Mills’s 2013 work, The League: The True Story of Average Americans on the Hunt for WWI Spies.