Johnson ignores a CIA report that challenges the domino theory.
The “domino theory” was a product of the Cold War and justified the U.S. involvement in helping smaller countries avoid communism. It was a belief that if one country fell to communism, then surrounding countries would “fall like dominoes”. The CIA report stated that Cambodia would be the only country to fall immediately but even if Vietnam and Laos fell to Communism, there were enough American Allies in the Pacific to dissuade N. Vietnam and China from aggression.
Johnson, who held strongly to the domino theory, chose to ignore this report, eventually committing over 500,000 troops to Vietnam to try to thwart the spread of Communism.