According to the domino theory American leaders feared that other surrounding countries would also become communistic countries falling like dominos.
When North Vietnam became Communistic it pressured South Vietnam. When South Vietnam became Communistic is put pressure on both Laos and Cambodia which also fell into the Communist camp.
The American leaders feared that as each country fell to communism it would put pressure on the next country until all of southeast Asia became Communistic.
Their fears were not irrational three countries did fall into the communistic camp but the domino effect did not continue into Thailand and what was then Burma.