What was the us policy that required the us to fight in the vietnam war

The U.S government viewed its involvement in the war as a way to prevent a communist of South Vietnam.This was part of the domino theory of a wider containment policy, with the stated aim of stopping the spread of communism world wide :source: https://en.m.wikipedia.org/wiki/Vietnam_War

It can provide a social code to protect each citizen's future economical earnings. and then obviously defending the country from invading countries.

It covered the territory of land across eurasia from northeastern europe to the pacific ocean.