What countries tried to imperialize Vietnam? What was the U.S. view on it?

Question

What countries tried to imperialize Vietnam? What was the U.S. view on it?

Answer 1

Imperialize?

Chinese emperors basically dominated Vietnam from (roughly) 200 BC to (roughly) 900 AD, but I assume you mean colonize. The French (a republic) colonized Vietnam from around the 1850s and at that time, US didn't express much of an opinion about that.

Imperial Japanese forces occupied (colonized?) Vietnam from 1941 to August 1945. I suppose you know that US fought Japan in the Pacific during WW II.

When France returned to Vietnam, the US backed France  during the First Indochina War (between Viet Minh and the French), which lasted until July 20 1954. China and USSR backed Viet Minh. It ended with a ceasefire negotiated in Geneva.