The United States Army and the U.S. Navy conducted joint operations in search of rat tunnels leading in from and near the coast line of the Bon Hai river. The rat tunnels were believed to be created by the 4th Division of the North Vietnam led by Pham Ngoc Thao. The tunnels were used in transport of supplies, and small attacks on forward American patrols. Secondary objective of Operation Beaver Cage was to infiltrate the tunnels, pushing the enemy out into ambushed locations.