The US got involved in Vietnam War because of their foreign policy of containment. This idea was focused on stopping the spread of communism on a global scale. In Vietnam, North Vietnam (a communist country) was threatening to over take South Vietnam (a democratic country). So the United States sent their military into help.
The fighting in Vietnam was not ideal for the American military. This is because the Viet Cong (North Vietnamese military) was not the standard military. They did not dress in uniform and used guerilla warfare tactics to keep the American military off balance. After almost a decade of involvement , the US implemented a policy of Vietnamization. This resulted in the gradual withdrawal of US troops under the Presidency of Richard Nixon.