You made a common mistake on history. You write: "Vietnam beat back the U.S. backed forces and the US left." The reality is that US beat back North Vietnam and its invading forces, and the US left.

Actually, US and US backed forces won, and then the US left because the left-wing took over Congress which ;turned victory into defeat.

It took the North Vietnamese two years to recover enough to be able to attack then conquer the South. During that time US aid, promised as part of the peace agreement, was mostly denied. Even without the promised aid, and with zero US military support, the South Vietnamese fought strongly against the tanks, aircraft and infantry of the invading North, but were overwhelmed.

Johnson screwed up the war, along with MacNamara (SecDef) and General Westmoreland. Nixon and Westmoreland's replacement, Abrams, defeated North Vietnam. North Vietnam's puppet army in the South (the so-called Peoples Liberation Army - or VC) was defeated and destroyed in the Tet Offensive of 1968.

Vietnam fell due to people like Chomsky telling falsehoods (knowingly or otherwise) and convincing the elite opiniomakers that the war was lost - even as it was being won. Chomsky is notorious for his anti-American screeds.