Because the modern military is primarily composed of volunteer enlistees, contract law is just as much of a consideration for military commanders as combat training or logistics. Every new soldier is contractually obligated to provide at least eight years of military service, even if only part of that time is spent on active duty. Eventually a soldier may become part of a reserve unit until he or she reaches that eighth year or voluntarily re-enlists. A stop-loss military policy can change all of those circumstances for certain soldiers during a time of war.
The date which officially establishes the end of a soldier's military service obligation is known as the end of terms of service or ETS date. Ostensibly, this ETS date is fixed and unassailable from the soldier's point of view. During a time of war or other emergency need for trained personnel, however, that ETS date can be overridden by presidential or congressional order through the Pentagon. This "stop-loss military policy" order can legally force a selected soldier to involuntary extend his or her military obligation up to six months after the end of a war.
Under a stop-loss military policy, a selected soldier, reservist or guardsman can also be prevented from transferring from a war zone to a new assignment, called a permanent change of station or PCS in military jargon. If a certain soldier's skills are deemed critical to the success of a combat mission, such as a helicopter pilot with battlefield experience, then he or she can be ordered to remain in a war zone instead of moving on to a new assignment in a more secure location.
The stop-loss military policy does allow some military personnel to voluntarily "separate" from active duty before the stop-loss order would take effect, but this action can only be taken after completing a often-involuntary set of other duties. Because the stop-loss military policy is clearly included in the fine print of a soldier's service contract, pursuing legal action to prevent an involuntary extension of military obligation or a forced return to combat conditions is extremely difficult.
The loss of trained soldiers in mission-critical positions following the Vietnam War spurred the United States Congress to pass legislation which created the original stop-loss military policy. The power to force a volunteer soldier to remain in a state of involuntary military service, however, was designed to be implemented only during times of war or extreme national emergency. Stop-loss was first used during Desert Storm, the first Gulf War. It has also been invoked during the Bosnian conflict and during the second Gulf War in Iraq.