The Vietcong, also know as The Peoples Liberation Armed Forces, fought for the National Liberation Front. The Vietcong were guerrilla soldiers that blended in with the South Vietnamese and struck only when they knew they could win and then would disappear and blend back in. The Vietcong developed what was known as rat tunnels which they used to travel in secrecy.
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http://www.vietnampix.com/popvc.htm