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In my country there used to be a mandatory military service or alternatively a longer civilian service; I choose the latter> I'm sorry that your country condoned slavery. These sorts of policies and laws will eventually be condemned universally by history once they fall out of fashion.
Tell that to the people in Ukraine, which has mandatory service:
* https://en.wikipedia.org/wiki/Military_service#Ukraine
For more open societies, the people themselves can decide what is part of the social contract. Some places have made the collective choice that citizens being able to defend against external threats is a good thing. Not everyone has friendly neighbours.
It is no different than taxation (IMHO): through their representatives, people have chosen that each person shall given up a portion of their income for the collective good. Under your logic taxation would be theft. Some societies have decided that labour is part of what needs to be given up.
* https://en.wikipedia.org/wiki/Social_contract