How was the vietnam war a war of imperialism?

The Vietnam wasn't really a war on Imperialism after the French got kicked out of South Vietnam. Okay, so here's a run-down of what happened:
So France had control of Vietnam in the 1800's and early 1900's. Then during WWll, Japan had control of Vietnam. When Ho Chi Minh and the Vietminh kicked out the Japanese in the 1950's, France swooped in again and took control of the Southern portion of Vietnam to expand their territory (This is what was considered Imperialism). The U.S. sent aid to France before France bailed and was like I'm done with this place. This is when it became a war to contain Communism. So, at the Geneva Conference, Vietnam was divided at the 17th parallel with the North, controlled by Ho Chi Minh, being communist and the Democratic South controlled by Ngo Dinh Diem.