What is the theme in In the Lake of the Woods by Tim O'Brien?

Ambition is a theme in the story. John sees himself as a political figure climbing the ladder and takes that self-image very seriously from a young age. When John is involved in a massacre in Vietnam, he is horrified by his own actions and by those of his fellow soldiers. When a fellow soldier suggests that they should tell a military officer of the atrocities that occurred, he refuses, realizing what his association with those events would do for his political career. John then takes this another step and when he finds himself at a desk job he begins falsifying records in order to further remove himself from the events. He goes so far as to promote and bestow medals upon himself, fluffing up his military career in order to benefit his political ambitions.