1. The term "fifth column" originated in Spain (originally quinta columna) during the early phase of the Civil War. It gained extreme popularity in the Republican media since early October 1936, and immediately started to make rounds also abroad. Exact origins of the term are not clear.

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Some authors consider it possible if not likely that the term has been invented by the Communist propaganda with the purpose of either raising morale or providing justification for terror and repression; initially it might have been part of the whispering campaign, but was later openly floated by Communist propagandists.[18] There are also other theories afloat.[19]

Some writers, mindful of the origin of the phrase, use it only in reference to military operations rather than the broader and less well defined range of activities that sympathizers might engage in to support an anticipated attack.[a]

Two 5th columns…theirs and ours…5:5