fifth column 

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 by LudwigVan October 1, 2004

fifth column 

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.

fifth column by takethathill August 22, 2006

Fifth Columnist 

A fifth column is a group of people who clandestinely undermine a larger group such as a nation from within. A fifth column can be a group of secret sympathizers of an enemy that are involved in sabotage within military defense lines, or a country's borders. A key tactic of the fifth column is the secret introduction of supporters into the whole fabric of the entity under attack.

Bobby: Not ironically, Fifth Columnist rhymes with Filthy communist.
Joe: You got me there, Bobby, I have no idea.
Bobby: Yeah, right.

Fifth Columnist by Chaotic_Neutral October 20, 2012