The Military Reserves are an organization that is associated with the military but is not in active duty.
The purpose of a military reserve is to create armed units that are trained and equipped to fight if they are needed, but without the cost and burden to the members of those units of being in the active duty military.
The United States military has an extensive reserve system. When a soldier, sailor, marine, airman, coast guardsman, or merchant mariner is discharged from the military, they often choose to enlist in the reserves. This allows them to return to civilian life. They must commit to, and attend, regular though not terribly time-consuming training. They must also commit to serving in active duty if their unit is called to war. In exchange for this commitment and work, they are paid.
This system has benefits for both the military and the servicemember.
All the branches of the military have reserves.
U.S. Army Reserve
U.S. Navy Reserve (also known as the U.S. Naval Reserve)
U.S. Marine Corps Reserve
U.S. Air Force Reserve
U.S. Coast Guard Reserve
U.S. Merchant Marines Reserve
In the U.S., the reserve sytem is different from the National Guards, which are affiliated with state governments as well as the federal government.
Military strength is a term that describes a quantification or reference to a nation's standing military forces or the capacity for fulfillment of that military's role.
For example, the military strength of a given country could be interpreted as the number of individuals in its armed forces, the destructive potential of its arsenal, or both.
Military force is a term that might refer to a particular unit, a regiment or gunboat deployed in a particular locale, or as an aggregate of such forces (e.g.