President johnson entered the vietnam war because he (

a. believed in the domino theory (

b. ran for office as a war dove (

c. underestimated the communist threat (

d. wanted to institute a realpolitik policy.

President Lyndon Johnson was a firm believer in the domino theory (so I'd say that the correct answer is a.). Basically, the domino theory said that if one country in a region came under the influence of communism, then the surrounding countries would follow in a domino effect. Because of this, and because of the advice from his advisors, the President took a more forceful approach to the Vietnam conflict.