On January 27th of 1916, in the middle of the First World War, conscription was introduced into the UK. Men between the ages of 18 and 41 were required to serve in the armed forces. The age was later raised to 51. Exceptions were allowed for those in vital industries, church ministers, or conscientious objectors.
Conscription itself is an ancient practice, imposed in ancient Egypt 27 centuries BC, as well as in Hammurabi's Babylon 4 millennia ago. Its modern form came in the late 18th century when revolutionary and Napoleonic France needed soldiers. It is a denial of liberty, done in the name of national security. Young men, and in some countries women, are forced to undertake military training and to serve in the armed forces. It interrupts careers and forces people into activity they might not freely undertake.
It was phased out in the UK in 1920, but reintroduced in 1939 as the prospect of war loomed. Its second incarnation lasted until 1960, when it was replaced by an all-volunteer army, which we still have.
The first peacetime draft, as conscription was called in the US, lasted from 1940 to 1973, when the US also moved to an all-volunteer army. There was a famous exchange before President Nixon's Commission on a volunteer army, when Milton Friedman and General Westmoreland confronted each other.
General William Westmoreland, testifying before President Nixon's Commission on an All-Volunteer [Military] Force, denounced the idea, saying that he did not want to command an army of mercenaries.
Milton Friedman interrupted him: "General, would you rather command an army of slaves?" Westmoreland got angry: "I don't like to hear our patriotic draftees referred to as slaves."
And Friedman got rolling: "I don't like to hear our patriotic volunteers referred to as mercenaries. If they are mercenaries, then I, sir, am a mercenary professor, and you, sir, are a mercenary general." And he did not stop: "We are served by mercenary physicians, we use a mercenary lawyer, and we get our meat from a mercenary butcher".
Despite the restrictions it imposes on personal freedoms, conscription remains in many countries today, with a typical period of maybe two years of service followed by several years as a reservist. Some today call for UK conscription to be reintroduced, perhaps for military service, or maybe for "community service." They should bear in mind that taking over people's lives for a year or two, and forcing them to do something they have not chosen to do, is a form of slavery. Milton Friedman was basically right.