With very few exceptions, employers are bound by the Uniformed Services Employment and Reemployment Rights Act of 1994 (USERRA) to reemploy and provide additional benefits to those employees who have performed military service as a member of the Army, Navy, Air Force, Marine Corps, Coast Guard, National
In order for an employee to qualify for these benefits, they must have held a civilian job and informed you that they were leaving the job to serve in the armed services. They must not serve for more than five years, and cannot have left the service under "dishonorable conditions." Finally, they must report back to you in a timely manner for reemployment.
More information on USERRA can be found at the U.S. Office of Special Counsel Web Site.