I am a retired Officer - the Army (can't speak for the other services) do "recall" people but it is usually on a volunteer basis and in specific MOS's
(skills) that are short for some reason.
When one enlists (Private Soldiers), is appointed (Warrant Officers) or is Commissioned (Officers) they do so for an 8 year obligation a portion of
which, usually 4 years is spent on active duty and the rest either in the active reserve (AR) or individual ready reserve (IRR).
Reservists drill once a month for 2 days and for a 14 day period usually in the Sumer.
Inactive Ready Reservists do not have to drill but are required to update their address and contact information on a regular basis. However, they are
sometimes offered the opportunity to go on active duty for short periods for training or a deployment if their specialty is needed.
In the event of a large scale mobilization they would first they send letters to those units with Soldiers who are in the AR (also perhaps the
National Guard) who hold the MOS in question and ask them to canvas the unit for volunteers. If they can't get enough that way they would tap into
the IRR in the same way.
They use these individuals to fill shortages in units who are about to deploy for key training or a real world mission.
Finally in the case of a major mobilization they might activate the members on the retirement roles. This is usually limited to those who have
retired less than 5 years ago as skill sets tend to atrophy and technology advances quickly.
They would use these retired individuals who are usually senior NCO's and Officers to staff and man stateside headquarters, training and recruiting
units so that those with the most recent training would be on the pointy end of the spear so to speak.
A full national mobilization (anything other than onzies and twozies in key roles) requires the order of the POTUS and approval of Congress plus the
Emergency Declaration. This is under Title 10 U.S.C. 12301.
So in summary; reservists, both AR and IRR are regularly offered the opportunity to augment units that are deploying especially those with unique or
shortage skills like linguists, air traffic controllers, EOD, intelligence, etc.
Hope that helps.
edit on 30/3/2011 by Golf66 because: (no reason given)