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.
