The US became involved in Vietnam for several reasons: to contain communism according to the Cold War policy, the Domino Theory that communism would spread in Asia if one country fell, and the French withdrawal from their colony of Vietnam. The Gulf of Tonkin resolution allowed the president to send troops without congressional approval. The US supported the anti-communist leader Diem in South Vietnam but later helped remove him. Despite initial advisors, the US increasingly committed troops under Kennedy and Johnson but failed to win, contributing to Johnson not seeking re-election. Nixon took office promising to end the war through "Vietnamization" while using tactics like Agent Orange and napalm bombings. Growing protests at home resulted from the Tet Offensive showing