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 alleged attacks on US destroyers. However, the US struggled against an elusive Viet Cong enemy hidden in jungles, without clear targets. Growing anti-war sentiment at home was exacerbated by the surprising Tet Offensive, though US losses were much lower than Vietnamese casualties. The US then began withdrawing forces through the Vietnamization program while still supporting South Vietnam.