The Vietnam War myth

[FM WarB]

"South Vietnam was free, sovereign, protected by its own military, and in control of its territory. When North Vietnam attempted direct cross border invasion in the Easter Offensive in the spring of 1972 anyway, US air power intervened massively and smashed the attempt."

If South Vietnam was protected by its own military, why was massive US airpower intervention required to turn back the North Vietnamese in 1972? Why, after three more years of standing up while we stood down did the ARVN collapse like a rotten house of cards without such intervention in 1975?