President johnson entered the vietnam war because he

believed in the domino theory

Explanation:

Domino Theory: "The theory that a political event in one country will cause similar events in neighboring countries, like a falling domino causing an entire row of upended dominoes to fall."

Since Vietnam got turned into a communist state, President Johnson believed that other neighboring states would be turned to communism. He didn't want other countries to convert to communism so he entered the Vietnam war to get Vietnam back to non-communism.

answer; /// i believe that the correct answer is (d)the huang he river valley;