The U.S. defeated North Vietnam during the Tet Offensive in 1968, was fully out of the fight in 1973, except for the Marines guarding the embassy. Congress lead by Ted Kennedy cut funding to Vietnam in 1975, then the North Invaded.
The U.S. Army already won the Vietnam War five years before Congress lost it. Academics, Hollywood, and the news have been blaming the Army ever since. It has taken a long time to start to correct the record.
I know that history well. I landed on my first tour in February 68 and spent a year in the Tet Counteroffensive the the 11th Armored Cavalry Regiment.