Elected U.S. president in 1960 after serving since 1953 as senator from Massachusetts, Kennedy was assassinated in November, 1963 -- three weeks after the murder of South Vietnamese President Ngo Dinh Diem. Vice President Lyndon B. Johnson stepped into Kennedy's shoes following the assassination. Many political and military analysts believed Kennedy -- had he not been killed -- would not have involved U.S. troops in the war between North and South Vietnam.