1. the term of copyright for the work has expired;
2. the author failed to satisfy statutory formalities to perfect the copyright, or
3. the work is defined as a work of the U.S. Government, although recent changes to the law allow for exceptions.*
As a general rule, most works enter the public domain because of old age. This includes any work published in the United States before 1923. Another large block of works are in the public domain because they were published before 1964 and copyright was not renewed. (Renewal was a requirement for works published before 1978.) A smaller group of works fell into the public domain because they were published without copyright notice (copyright notice was necessary for works published in the United States before March 1, 1989).
*For more information about Copyright and issues affecting the U.S. Government, see CENDI's Copyright and Intellectual Property workgroup's web page.
There has been an amendment to 17 USC §105 as it pertains to works by civilian faculty at selected defense educational institutions, including NPS.
For up to date information, see the NPS Office of Counsel page. (Intranet only; Login with NPS credentials)