War is a nasty business, based on what I’ve read over a lifetime. Civil war is worse still. It can pit parent against child, sibling against sibling. It fuels suspicion of one’s fellow-citizens. In Summer 1936, civil war broke out in Spain. Although often seen as a prologue to the Second World, the Spanish Civil War was a primitive affair. Not a lot of tanks, or aircraft, or trucks. Marching up toward Madrid, the Nationalist (rebel) commander Emilio Mola divided his troops into four columns to better live off the barren land. He told the foreign correspondents accompanying his army that he had a “fifth column” of sympathizers inside the city which would support his troops. The phrase “fifth column” quickly passed into the common lingo of the era.
In 1938, Austrian Nazis supported the German take-over of Austria. Sudeten Germans around the frontiers of Czechoslovakia agitated for a German taker-over, obviously at the behest of Berlin. Poles-of-German-ancestry demanded free dome from alleged “persecution.”
In Spring 1940, the Nazis unleashed their “Blitzkrieg” on Western Europe. Denmark, Norway, Holland, Belgium, and—astonishingly—France collapsed. The idea that a powerful state like France could be beaten in weeks boggled the mind. “Collaborationist” regimes, or at least individual “collaborators,” sprang up in many places. The reactionary French Vichy government and the puppet-state in Norway headed by Vidkun Quisling offered prime examples. It soon became an article of faith in Britain and the United States that pro-Nazi “fifth columnists” had undermined their own society in the conquered countries.
Both in Britain and in the United States a hunt for “fifth columnists” soon began. In Britain, the new prime minister quickly put a stop to the left’s demands for prosecution of “the Guilty Men” who had supported appeasement. Only a handful of obvious candidates were detained (Oswald Mosely, the leader of the British Union of Fascists, for example).
It proved to be very different in the United States. There an increasingly bitter debate began over American policy toward involvement in the global conflict. Lynne Olson has argued that the Roosevelt Administration engaged in a campaign of vilification against the leading exponents of “isolationism.” The most notable target was Charles Lindbergh. The “Lone Eagle,” once America’s most admired person, suffered repeated, vitriolic attacks in the press and by FDR’s surrogates. (Interior Secretary Harold Ickes looks worse than he once did.)
Subsequently, after Pearl Harbor, the federal government criminalized Japanese ancestry on the grounds that such people were inherently disloyal. Shrugging off that incident, Americans then launched themselves on an anti-Communist witch-hunt in the later Forties and in the Fifties. As Arthur Schlesinger the Lesser wrote in 1949: “the special Soviet advantage—the warhead—lies in the fifth column; and the fifth column is based on the local Communist parties.” The down-side of this appeared in “black-listing” (See: “Trumbo”) and “McCarthyism.” Much ignored is the reality of Soviet penetration of the US government.
So, the fear of disloyal Americans is nothing new. Most often, it’s been misplaced. That will not stop the idiots and hysterics.
 See how political correctness has watered down my prose?
 Ernest Hemingway wrote a play called “The Fifth Column.” On Mola, who knew something about civil war, see https://en.wikipedia.org/wiki/Emilio_Mola
 It was hard to argue with a guy who had vocally opposed appeasement when he draws a veil over the past.
 Lynne Olsen, Those Angry Days: Roosevelt, Lindbergh, and America’s Fight Over World War II, 1939-1941 (New York, Random House, 2013).
 EffaBeeEye Director J. Edgar Hoover, seems to have thought the charges a crock. He headed American counter-intelligence. See: https://en.wikipedia.org/wiki/Internment_of_Japanese_Americans