The only war the United States has ever lost. After WW2 and the rise of the USSR, North Vietnam became a communist region. The US was afraid that if South Vietnam fell to communism, other small countries would also become communist. At first they sent advisors to the South's army but they needed extra support so US troops were sent. It was a war of attrition, victories were won not by total area gained but by body count. It was a very nasty war, with a lot of hatred by the American public. Over 50,000 american soldiers died, as opposed to the 2 million North Vietnamese killed. A lot of American pilots were shot down and captured as POWs and sent to camps, such as the famous "Hanoi Hilton" that became a symbol of the barbaric Vietcong. Contrary to popular belief, the US never lost a battle in Vietnam. Nixon decided to pull out due to the negative opinion of the war and to secure his reelection calling it "Peace with honor". The State Dept lost the war folks, not the military.
My ROTC instructor was a Huey pilot in the Vietnam War.