Fifth Column
According to Wikipedia a Fifth Column or Fifth Columnist is an individual or a group of people who clandestinely undermine a larger group such as a nation from within. The term was coined by radio announcer Emilio Mola in 1936.
In the Marvel Universe it's usually a blanket term for groups that attacked America and other Allied countries during World War II, and later during the Cold War, to identify groups from Communist countries trying to undermine Democratic countries.
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