Encyclopedia Britanica:
Fifth column, clandestine group or faction of subversive agents who attempt to undermine a nation’s solidarity by any means at their disposal. The term is conventionally credited to Emilio Mola Vidal, a Nationalist general during the Spanish Civil War (1936–39). As four of his army columns moved on Madrid, the general referred to his militant supporters within the capital as his “fifth column,” intent on undermining the loyalist government from within.
A cardinal technique of the fifth column is the infiltration of sympathizers into the entire fabric of the nation under attack and, particularly, into positions of policy decision and national defense. From such key posts, fifth-column activists exploit the fears of a people by spreading rumours and misinformation, as well as by employing the more standard techniques of espionage and sabotage.
Interesting detail that the “Nationalists” in the Spanish Civil War were the Fascists led by Gen Francisco Franco and the “Loyalists” (also known as “Republicans”) were the Communists support by the Soviet Union.
So the term Fifth Column was invented by Nationalists/Fascists who infiltrated the Loyalist government to destroy it from within. The average Democrat in the US probably doesn’t realize how much their party has been taken over by Fascists.
“A cardinal technique of the fifth column is the infiltration of sympathizers into the entire fabric of the nation under attack and, particularly, into positions of policy decision and national defense. From such key posts, fifth-column activists exploit the fears of a people by spreading rumours and misinformation,....”
Precisely what the CCP has done with the Democrat Party