The US became involved in Vietnam to prevent South Vietnam from falling to communism, fearing this could lead other Southeast Asian countries to also fall (domino theory). While initially sending advisors, the Gulf of Tonkin Resolution allowed greater US action after two American destroyers were attacked. There was division at home between those favoring and opposing the US presence in Vietnam. Fighting was difficult for the US due to jungle terrain, lack of large targets, and challenges identifying Viet Cong opponents. A surprise Tet Offensive by Vietnamese Communists in 1968 despite a ceasefire caused heavy casualties on all sides. The US later began gradually transferring responsibility for fighting to South Vietnam through a Vietnamization process while still providing support.