Throughout 1969 and into 1970, the North Vietnamese army was making increasing use of sanctuaries in Cambodia, both to supply troops in the field and for use as staging areas for offensives into South Vietnam. Communist violations of Cambodia's sovereignty had begun as early as 1965.
In March 1969, President Nixon ordered unannounced bombing runs into Cambodia to weaken the enemy's supply routes and storage areas. Shortly after the first such attack took place, the North Vietnamese acted on the President's proposal to initiate a secret negotiating channel. While this action met with substantial success, it became clear that a more intensive operation would be needed to counteract North Vietnam's massive abuse of Cambodian neutrality to support its war machine.
On May 14, 1969, President Nixon made his first major address about Vietnam over nationwide TV and radio. His peace proposal went beyond anything previously offered by the United States.
Speech: This is not an invasion of Cambodia