Which event is associated with the emergence of the term "fifth column"?

General Mola was approaching Madrid with 4 columns of troops and predicted that the city would fall when they were helped by a fifth column secretly working in the city

What are the 1st four coulmns, if this is the fifth?!😵‍💫

We have a fifth column operating here in the U. S. to undermine elections. 😐

Player #53553731, it also applies to politics

Very vague reference to the origin of the term.