South Vietnam won the war – at first

(1)  If we believe Richard Sorely’s 1999 book, A Better War, by mid-1972, the poor bedraggled South Vietnamese villagers, with about half the obsolescent American weapons they could have used, had so reduced their poor bedraggled Viet Cong counterparts that an American ambassador could travel the countryside without escort.
https://www.amazon.com/Better-War-Unexamined-Victories-Americas/dp/0156013096

(2)  This in turn made it impossible for Northern main forces to conceal their caches of food, medicine and ammunition delivered down the Ho Chi Minh trail– their main force soldiers were eating grass.  

(3)  Which in turn prodded North Vietnam to send every last soldier it had left at home – 200,000 – down south in an old fashioned conventional tank and artillery assault in a desperate attempt to relieve -- thrown back with 50% casualties.

The defending infantry were exclusively Vietnamese -- aided by American logistical, air and intelligence support.

Three years later North Vietnam blitzed its way through South Vietnam in three months.  With three more years to shape up, the South should have been even more ready.  What happened?  No American logistical, air, intelligence and financial support.  

We had taken our bat and ball and gone home.  After losing 60,000 American lives and spending hundreds of billions of dollars we wouldn’t even give South Vietnam the money to finish the job …

… craziest thing I’ve seen in my life.

Cannot fault Southern spirit:
“On the road to Saigon at Xuan Loc, an ordinary South Vietnamese unit, the 18th Division, fought battle-hardened North Vietnamese regulars backed by tanks, trucks, and artillery to a standstill for two full weeks. Not only did those South Vietnamese soldiers take heavy casualties, with more than a third of their men killed or wounded, but they held their positions through those long days of “meat-grinder” combat until the enemy had to circle around them to reach the capital.”
https://www.nakedcapitalism.com/2021/09/the-winner-in-afghanistan-china.html

According to Frank Snepp’s 1977 book, Decent Interval, we had someone on the North Vietnamese politburo and they had voted to throw in the towel (not sure the timeline here) – but when they in time caught on that the South was rationing artillery rounds, etc., they decided to start up again – with the well known result.
https://www.amazon.com/Decent-Interval-Frank-Snepp-dp-0394407431/dp/0394407431/ref=mt_other?_encoding=UTF8&me=&qid=1633025165    

The decision year.  In 1965, our knees were still knocking over two little countries almost taking over the world 20 years earlier.  Now, the two biggest countries in the world were (very vocally) coming to get us (“We will bury you”).  It may look 10 X more dangerous than it should when it’s coming at you, but at that time but we were plenty scared (think Korea).