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

The author makes the note in the opening pages of the book, alerting the reader to the fact that there are "references to actual places, people, and events, it must be read as a work of fiction." There are many references to the real people and places. The war in Vietnam is a prime example of the "actual" events while the massacre of Indians at the hands of the American General Custer is another. The use of some real events and places helps lend credence to the story overall.