Short answers, pulled from the story.
Conscription is the compulsory enlistment of individuals into national or military service enforced by law. The practice dates to antiquity; around 1791 BC the Babylonian Empire under Hammurabi used a system called Ilkum requiring eligible subjects to serve in the royal army during wartime and provide state labor in peacetime.
The French Revolution introduced mass national conscription, known as levée en masse, to enable the Republic to defend itself from European monarchies. Deputy Jean-Baptiste Jourdan gave his name to the act of the 5th of September 1798, and between 1800 and 1813 more than 2.6 million men were inducted into the French military under this system.
Norway was the first, introducing female conscription in 2015 as the first NATO member and first country in the world to draft women on the same formal legal terms as men. Sweden followed in 2017 when it reinstated compulsory service for both genders, becoming the second country to do so.
The United States has not enforced conscription since 1973. However, every male resident is still required by law to register with the Selective Service System within 30 days of his 18th birthday. Men who fail to register can be barred from federal financial aid, federal employment, citizenship applications, and in some states a driver's license.
On the 7th of January 1918, the Supreme Court unanimously upheld the constitutionality of the draft in Arver v. United States. The Court held that the Constitution's grant of power to Congress to declare war and raise armies included the reciprocal obligation of citizens to render military service when called.
Several countries provide alternative service paths for conscientious objectors. Finland offers siviilipalvelus, a civilian alternative, and Austria and Switzerland offer Zivildienst, a compulsory community service. Sweden allows conscientious objectors to choose weapons-free civil defense service.