Why was the Vietnam War unsuccessful for the United States?

US containment failed because of the Communist Superpowers (their existence) and the geography of the land being fought on. The battlefield (South Vietnam) could NOT be isolated from enemy reinforcements and supplies. As it had been during the Korean War (Korea was a Peninsula, easily controlled by the US Navy). Controlling the enemy supply routes would have involved other countries. Meaning; widening the war. Widening the war, could have caused an intervention of the one or more of the communist superpowers (as DID happen during the Korean War). So it was "fish" or "cut bait"; we cut bait.