Fifth-column

noun
1.
a group of people who act traitorously and subversively out of a secret sympathy with an enemy of their country.
2.
(originally) Franco sympathizers in Madrid during the Spanish Civil War: so called in allusion to a statement in 1936 that the insurgents had four columns marching on Madrid and a fifth column of sympathizers in the city ready to rise and betray it.
noun
1.
(originally) a group of Falangist sympathizers in Madrid during the Spanish Civil War who were prepared to join the four columns of insurgents marching on the city
2.
any group of hostile or subversive infiltrators; an enemy in one’s midst
n.

1936, from Gen. Emilio Mola’s comment at the siege of Madrid during the Spanish Civil War that he would take the city with his four columns of troops outside it and his “fifth column” (quinta columna) in the city.

People willing to cooperate with an aggressor against their own country. The term originated in a remark by Francisco Franco, the Spanish dictator, that he was marching on Madrid with four columns of troops, and that there was a “fifth column” of sympathizers within the city ready to help.
A secret subversive group that works against a country or organization from the inside, as in The government feared that there was a fifth column working to oppose its policies during the crisis. This term was invented by General Emilio Mola during the Spanish Civil War in a radio broadcast on October 16, 1936, in which he said that he had una quinta columna (“a fifth column”) of sympathizers for General Franco among the Republicans holding the city of Madrid, and it would join his four columns of troops when they attacked. The term was popularized by Ernest Hemingway and later extended to any traitorous insiders.