Being compelled to do something isn't slavery if you're being compelled to perform your social duty. That's the point of government--you have a negative duty to not do certain things (kill other members of your society or steal things from them), but historically most cultures have also recognized a positive duty to do certain things (pay taxes, defend your society against its enemies). It's an unpopular argument now, and I don't necessarily buy it, but it's still around--implicitly, at least, in every country's naturalization system, where new citizens explicitly accept the responsibilities of citizenship.
(In practice, of course, conscription has regularly been used to raise armies for offensive wars against imaginary threats. That's a betrayal of the government's duty to its people. In practice, this makes conscription a very dangerous tool to leave available. However, a degenerate case does not prove the general case--in terms of moral principles, just because it is immoral to use conscription to attack an innocent neighbor does not mean it is immoral to use conscription to defend against an aggressive enemy.)
(In practice, of course, conscription has regularly been used to raise armies for offensive wars against imaginary threats. That's a betrayal of the government's duty to its people. In practice, this makes conscription a very dangerous tool to leave available. However, a degenerate case does not prove the general case--in terms of moral principles, just because it is immoral to use conscription to attack an innocent neighbor does not mean it is immoral to use conscription to defend against an aggressive enemy.)