One of the definition of war crimes includes attacking a neutral country. The US didnt declare war or even said anything until the NYTimes exposed their secret bombing campaign in Cambodia. Prior to that the Cambodians complained at the UN, but the bombings were denied by the US even then.
The bombing was also indiscriminate. More Cambodian civilians died because the US didnt even know where the Vietnamese were other than along the Ho Chi Minh Trail.
And here was Nixon's defense of the bombing:
Following months of secret U.S. bombings on Communist bases, American ground troops were deployed to northern Cambodia on April 28, 1970.
www.history.com
So the reason he claimed that he did the bombing was not to destroy the enemy, it was to delay them so he could pull out American troops out of Vietnam gradually. He killed tens of thousands of innocents for political, not military purposes.
People to this day are still being crippled by leftover US ordinance in Cambodia.