The National Defense Service Medal (NDSM) was first intended to be a "blanket campaign medal" awarded to service members who served honorably during a designated time period of when a "national emergency" had been declared during a time of war or conflict. It may also be issued to active military members for any other period that the Secretary of Defense designates.

To date, the NDSM has been awarded for four specific time periods, which roughly correspond to the Korean War (1950-1954), War in Vietnam (1961-1974), Gulf War (1990-1995), and the Global War on Terrorism (2001 to 2022).