Five US Presidents were involved in the Vietnam War from 1955 to 1975 - Dwight Eisenhower, John F. Kennedy, Lyndon Johnson, Richard Nixon, and Gerald Ford. Eisenhower first sent military advisors to help train South Vietnam's army. Kennedy escalated US involvement by sending more troops as advisors. Johnson further increased troop levels after the Gulf of Tonkin incident and launched bombing campaigns of North Vietnam. Nixon pursued a policy of "Vietnamization" to gradually withdraw US troops by strengthening South Vietnam's forces. Ultimately, the US was unable to defeat North Vietnam and withdrew from the country in 1975.