From NBC's Jim Miklaszewski
If the U.S. government shuts down, all of the approximate 2.5 million active duty forces will be required to report for duty.
The men and women in uniform will not be paid immediately for each day of service, but will be paid retroactively once the government is back up and running.
Department of Defense civilian workers considered essential will also be required to work, and, like the military, would be paid rectroactively once the shutdown is over.
The remaining Department of Defense civilians would be furloughed, which means by law they cannot report to their job, would not be paid for days missed, and not paid retroactively unless Congress approves the back pay.
During a shutdown Defense officials and the military are required to provide for "safety, protection of human life, and defense of national security."
That's a very broad definition which means deployed military forces, and all military and civilians working in health care, child care, food services education, training, legal and finances are required to report to work.