What is the main conflict in Undaunted Courage by Stephen Ambrose?

In his introduction to Undaunted Courage, Stephen E. Ambrose writes that he feels "privileged to have had the opportunity to spend so much time with Meriwether Lewis." With this expansive work, which he had wanted to write for twenty years, Ambrose takes the opportunity to unfold the Lewis and Clark drama. He makes it come alive, even for those readers who, unlike Ambrose, have never followed the Lewis and Clark trail.