No it was not inevitable from the standpoint of the people of Vietnam who juast wanted to have a united country not divided between the north and south but it was inevitable from the drive of the American government to control SE Asia for political and economic reasons, including anti-communism and also to fuel the military industrial complex and sell arms. When the French were defeated at the battle of Dienbienphu then the US government saw its' chance to have another colony in SE Asia and try to control a country in its competition with the Soviet Union ie division of the world by the 2 superpowers of the time.