In some countries in Europe (Germany is one) and certain other places like South Korea, you owe a year of "service" after you turn 18 which can be military or something else like volunteer work. For South Korea this was particularly important because they wanted a large reserve that already had at least some training, in case of invasion by the North.
The bargain is that college is free, however. (At least in Europe)
This could be where we are headed with Bernie Sanders plan.