Final answer:
President Lyndon Johnson's decision to increase U.S. involvement in Vietnam resulted in a divided American society, economic strain, and heavy casualties, leading to eventual U.S. withdrawal and the unification of Vietnam under Communism.
Step-by-step explanation:
A significant result of President Lyndon Johnson's decision to increase U.S. military involvement in Vietnam was a deeply divided American society, with widespread protests and growing opposition to the war. The involvement, beginning with the Gulf of Tonkin Resolution, saw troop numbers increase to over half a million. Escalation of the war effort led to heavy causalities, with 58,000 U.S. troops and over a million Vietnamese deaths. The policy also strained the U.S. economy and, by fueling a credibility gap between the government and the public, created social unrest. The eventual withdrawal of U.S. forces in 1973 led to the fall of South Vietnam and the unification of Vietnam under communist control in 1975.