Answer:
He explained in a number of ways.
Step-by-step explanation:
When Johnson was sworn in after the assassination of Kennedy, US involvement in Vietnam rapidly escalated. Within 2-3 years there were over 500,000 American soldiers in Vietnam.
Johnson rationalized it in terms of the Cold War and Domino Theory. Stopping the spread of communism and checking Soviet and Chinese involvement in South East Asia and honouring commitments made by the US government particularly by Kennedy.
However this was despite the fact that privately, Johnson thought even at the beginning of his presidency, Johnson thought it was a losing cause. After Tet in 68, and the turning of public opinion Vietnam broke Johnson and he did not seek re-selection by the Democratic Party for that year's Presidential elections.