Question

why did america have to fight on vietnam?

Answer 1

Gradual withdrawal of U.S. ground forces began as part of "Vietnamization", which aimed to endAmerican involvement in the war while transferring the task of fighting the communists to the South Vietnamesethemselves and began the task of modernizing their armed forces.
Hope it will help you

Answer 2

American don't want the influence of Japan or China, that's why they have to fight on Vietnam!!!