This is an organization of a nation's armed forces for active military service in time of war or other national emergency?

The correct answer is "the Draft," except the United States no longer has one. Under current law, all male U.S. citizens are required to register with Selective Service within 30 days of their 18th birthday. In addition, non-U.S.-citizen males between the ages of 18 and 25 (inclusive) living in the United States must register. The draft was abolished on 23 January 1973, whereupon America went to the All-Volunteer military that defends America today.