The Great Impostor

The Great Impostor is a 1961 movie based on the true story of an impostor named Ferdinand Waldo Demara.

As he is arrested by the Coast Guard on an island in New England, a man born as Ferdinand Waldo Demara but known by many other identities recalls the events that brought him to this point.Demara quit high school as a boy and joined the Army. He wanted to become an officer, but his lack of education worked against it. On a whim, he fakes a set of credentials and becomes a U.S. Marine. ........

Source: Wikipedia