It's forced military slavery.
Ugh.
What is with the continued use of this metaphor? It's insulting to those who actually had to endure the horrors of slavery. Slaves were held for their entire lives, forced to work without pay, beaten, raped, killed. This is far worse than the contemporary "definition" of forcing people to do something.
Poor, but the idea of compulsory national service can have some merit as it can be a means to churn together the social classes and avoid them stratifying.
Many implementations of compulsory service (although certainly not all of them), are classist in nature, the draft during the Vietnam War being the greatest example of this.