"The U.S. under Nixon invaded Cambodia, which had been a neutral power during the war in Vietnam. "
A neutral power? They allowed the NVA to use Cambodia as a supply route and staging site to attack SVN for years and that is neutral?
"neutral power? They allowed the NVA to use Cambodia as a supply route and staging site to attack SVN for years and that is neutral?"
This is true. The Cambodian Royal Goverment was aware that the NLF and North Vietnamese Army were using staging areas inside their country to attack U.S. forces.
However, does this legitimitze the terrible destruction of an nation?
In the historical view that the bombing and subsequent invasion of Cambodia (and ensuing bloodshed) of that action-- no.
Furthermore, the invasion of Cambodia in 1970-71 was out of desperation by the Nixon admininstration. The U.S. had lost the war long before that. The U.S. would be out of Vietnam by 1973, and in 1975-- the South Vietnamese regime collapsed.