The "Agreement on the Cessation of Hostilities in Vietnam”, known as the 1954 Geneva Accords, was signed in July 1954 dividing Vietnam at the 17th Parallel. The Accords also set the borders of Southeast Asia, with Laos and Cambodia emerging as independent nations. The agreement established four separate nations, the Republic of Vietnam (RVN), the Democratic Republic of Vietnam (DRV), the Kingdom of Laos, and the Kingdom of Cambodia.
According to the terms of these Accords, which were worked out after the defeat of the French at Dien Bien Phu, there would be:
The North Vietnamese Viet Minh guerrilla forces that defeated the French were ordered to move above the 17th Parallel and French-led forces were told to stay in the south. Pending reunification, Vietnam's two separate nations would refrain from joining any military alliance. No foreign military bases could be set up and no new foreign military equipment or personnel could be brought in.
The demarcation line was not intended to be a political boundary. The division was supposed to be temporary until free elections could be held in 1956 to reunify all of Vietnam. However, not long after the Geneva Conference, North Vietnam and South Vietnam took shape on either side of the 17th parallel with Ho Chi Minh the leader in the north and Ngo Dinh Diem becoming the leader in the south. Neither Vietnam nor the United States had signed the Accords. Diem, believing that Ho Chi Minh would win the popular vote, refused to agree to elections. The partition became permanent.
U.S. President Eisenhower supported the division of North and South Vietnam. There was an exodus of about 1 million northerners, many of them Catholics, to South Vietnam. About 100,000 people moved in the other direction.
Nevertheless, the geographical area known as Southeast Asia was divided into four separate independent nations. Shortly after the Accords, when the North Vietnamese Army sent its agents across the 17th Parallel to support and build the communist insurgency of the Vietnamese communists, known as the Viet Cong, it was interfering in the affairs of another sovereign nation.
Worse, when the NVA invaded and occupied the eastern, mostly uninhabited part of Laos in the late 1950s in order to build its logistical network, known as the Ho Chi Minh Trail, it was invading the sovereign nation of the Kingdom of Laos. The NVA used the Ho Chi Minh Trail to send troops and war materiel to the RVN to overthrow the sovereign government there.
The troops and war materiel were hidden in a triangular area bordering Laos, Cambodia, and the RVN. Thus, the NVA also invaded and occupied the sovereign nation of the Kingdom of Cambodia. Neither Cambodia nor Laos was capable of thwarting the NVA occupation and use of their country and did not try since both areas were lightly inhabited.
You may read some books and see some documentaries that fail to mention that North Vietnam was a hostile invader of the Republic of Vietnam, the Kingdom of Laos, and the Kingdom (later Republic) of Cambodia. They fail to mention that the North had no right to try to unify North and South Vietnam nor invade Laos and Cambodia and foster the creation of communist governments there.
When one acknowledges this truth one wonders why this is not written into what appear to be alternative histories.