Fifth column
Etymology
Coined during the Spanish Civil War in 1936, when the Nationalist general Emilio Mola said four military columns were advancing on Madrid while a "fifth column" (Spanish "quinta columna") of sympathizers waited inside the city to rise up. The phrase spread rapidly through the coverage of the war.
noun phrase · /FIFTH KOL-um/
A hidden group within a country or organization that works secretly to undermine it in favor of an outside enemy.
What it names
The enemy within — the fear or reality that some of your own ranks are quietly working for the other side.
Example
Wartime paranoia about a fifth column led to sweeping and often baseless suspicion of immigrant communities.
Related idea
Where a quisling is a visible collaborator running things openly, a fifth column is by definition covert — a concealed network whose danger lies precisely in not being identified.
See also