Short answers, pulled from the story.
The British Army's half-pay system of the 18th century provided a pool of trained officers who remained civilians during peacetime but stood ready for wartime service. This arrangement functioned as an early reserve force without that specific designation.
Military reformer Gerhard von Scharnhorst introduced a new system that gave recruits a brief period of training which could be expanded during wartime. The Prussian army was limited to a maximum of 42,000 men under these harsh terms following the Treaties of Tilsit.
Countries including Colombia Israel Norway Singapore South Korea Sweden and Taiwan require reserve service for a number of years after completing national service. Switzerland and Finland utilize conscription systems where citizens who have completed active duty remain subject to mandatory mobilization in wartime.
Reservists are usually more economically effective than regular troops since they are called up as needed rather than always on duty. Their cost is limited to training and occasional deployments while many view voluntary training as merely supplemental income or a hobby making them inexpensive to maintain.
The United States maintains multiple reserve components including the Army Reserve Air Force Reserve Marine Corps Reserve Navy Reserve and Coast Guard Reserve. The National Guard serves as both state and federal forces across the country alongside these specific branches.