Was the idea that if vietnam became communist, all of se asia would as well.

Explanation:

Domino theory alleged that the spread of communism in Korea and Vietnam would result in other countries in Asia falling like dominoes to the influence of communism.

Domino theory was related also to the US foreign policy of contaiment -- seeking to contain communism where it was and not let it spread.

For more information and illustration, see this answer I posted on -

when world war i broke out across europe in 1914, president woodrow wilson proclaimed the united states would remain neutral, and many americans supported this policy of nonintervention. however, public opinion about neutrality started to change after the sinking of the british ocean liner lusitania by a german u-boat in 1915; almost 2,000 people perished, including 128 americans. along with news of the zimmerman telegram threatening an alliance between germany and mexico, wilson asked congress for a declaration of war against germany. the u.s. officially entered the conflict on april 6, 1917.

it would be a. that is the only one i could think of