The North Vietnamese took out a massive loan from the Soviets to purchase the arms needed for the invasion in 1975. If we had provided a similar loan to South Vietnam, they might have held on. But Congress cut off all aid, including loans, to make sure that North Vietnam would win. Just how big was the loan? The Vietnamese hadn't paid it off 20 years later. They settled up with the Russians for lease payments the Russians hadn't been making for the Cam Ranh Bay facilities.
And the Left really doesn’t like talking about that, nor about the way the friendly communists brutalized the people of the South and opened the way for the Killing Fields of Cambodia.