Throughout the war, the tunnels in and around Củ Chi proved to be a source of frustration for the U.S. military in Saigon. The Viet Cong had been so well entrenched in the area by 1965 that it was in the unique position of being able to control where and when battles would take place. By helping to covertly move supplies and house troops, the tunnels of Củ Chi allowed North Vietnamese fighters in their area of South Vietnam to survive, help prolong the war and increase U.S. costs and casualties until the eventual withdrawal in 1972, and the final defeat of South Vietnam in 1975.