Final answer:
The requirement for registering with the U.S. government for military service is a legal duty for all male citizens between the ages of 18 and 25. This registration is through the Selective Service System, which has changed over time but today does not include active duty unless there is a draft.
Step-by-step explanation:
The requirements for registering with the government for military service in the United States are a legal duty for all male citizens between the ages of 18 and 25, according to the Selective Service System. While actual military service is voluntary in the U.S. unless there is a draft, registration for Selective Service is mandatory. This system was established to ensure that the U.S. could mobilize forces quickly in the case of national emergencies. In historical contexts, various acts such as the Selective Service Act of 1917 and 1940 required men of certain ages to register for potential military service, with the age ranges and requirements changing over time. Recently, there have also been propositions to include females in the Selective Service registration process.