South Vietnam was considered an ally of the US.
South Vietnam
South (RVN-Republic of South Vietnam)
North Vietnam started the war against South Vietnam; the US went to the aid of the South.
The US didn't invade South Vietnam. The Republic of South Vietnam asked for US help as they couldn't stem the Red Tide (communist infiltration).
There IS no North and South Vietnam, only Vietnam. After the US withdrew combat forces from South Vietnam, North Vietnam violated the peace agreement, invaded and conquered South Vietnam. Game over.
US, New Zealand, Australia, South Korea, South Vietnam, Thailand, Philippines.
north Vietnam took over south Vietnam
The US was an ally of South Vietnam; it was up to South Vietnam to repel the invasion from the north, with massive military assistance from the US (which, of course, ultimately failed).
Simple, the US.
Nixon.
The US was trying to preserve the Republic of South Vietnam; an effort that failed.