A main event of President Johnson's administration was: the failed Bay of Pigs invasion conflict with Russia over missiles in Cuba active U. S. involvement in Vietnam

The answer is U. S. involvement in Vietnam. Before the assassination of President Kennedy, Kennedy had plans for withdrawing American troops from Vietnam. When Johnson became president after Kennedy died, he increase the number of U. S. troops in Vietnam. In addition, he launched a bombing camping in the North. Despite increase of troops and a relentless bombing campaign, the North continued to pour into the South through underground tunnels and using guerilla tactics against American soldiers. After the Tet Offensive, Johnson decided to slowly withdraw troops from the country. Later the North conquered the South and Vietnam became one country after communist rule.