A group within a state that attempts to subvert and weaken the state in order to assist an enemy. First used during the Spanish Civil War.
The characterization of leftists in the United States as a Communist 'fifth column' by the McCarthyist witchhunters survives today in post-9/11 America; only now Arabs and Muslims are blacklisted.
Fifth column is a group of people invading a country in a stealthy way. If everything goes well in a period of time the numbers outweigh the native people then the fifth column rise up and and take over the territory or country.
Fifth column examples: Muslims into England, Spain & France. Mexicans into the United States, California, Texas & New Mexico. Fall of the Roman Empire.
Mexicans are the fifth column.