Suppose Belgium is being attacked by a vicious enemy who is particularly targetting civilians—especially children—in a terroristic campaign. Currently, Belgium has an all-volunteer army. However, experts with excellent predictive track records estimate that conscripting 200,000 men for about a year will save almost as many lives, mainly those of children. These 200,000 draftees would be subject to the rigors and hardships of a tough year-long military campaign. Surprisingly, however, due to recent improvements in personal armor, the campaign is expected to result in fewer than 100 deaths of draftees.
There is nothing morally objectionable about the government having such a draft and those called up for it would have a moral duty to serve. Note that the projected death-rate expected from the campaign is about 50 times lower than the US military death-rate in WWII. Such a low death-rate makes this draft close to obviously right.
Now compare this to Judith Jarvis Thomson's arguments—like the famous violinist one—for abortion. In the Belgian draft case, in pregnancy and in Thomson's cases, people lose significant aspects of their ordinary freedoms and capabilities, and do so for the sake of saving the lives of others. I think the draft case underlines that Thomson's cases underestimate the degree to which we can be legitimately morally required to make significant sacrifices to save the lives of others.
Notice, too, three differences between the draft and violinist cases. While the particular people saved by the draft in my story are primarily children, and hence not the draftees themselves, the practice of instituting a draft in such dire wartime circumstances is one that all can potentially benefit from. If I am to be drafted and save the life of some child and I am considering running off, I should reflect that it's just a matter of my luck that the invasion happened now rather than when I was a child and when others would have been drafted to protect me. I need to do my bit or else I will be a freerider. It is not so in the violinist case. I am not a famous violinist. People are not at all likely kidnap anybody to provide life support for me! The pregnancy case here is like the draft case, but even more so. For we all not merely potential beneficiaries of the practice, but actual beneficiaries, since we all came into existence through pregnancy.
Second, in the draft case there is a not insignificant chance that the child whose life one's being drafted will save will be one's own child, while the violinist is a stranger. But in the case of pregnancy the child is almost always one's own (the exception is in cases of surrogate pregnancy). Again, the pregnancy case is more like the draft case, and even more in that direction.
The third difference may seem to play in a different direction, however. Both the pregnancy and the violinist cases involve direct use of one's internal organs. But in the draft, one's womb and one's kidneys are not being drafted—it is the use of external organs, like hands and legs, that is required of one. While there may be a small difference along these lines, I think the difference is not particularly significant. The soldier fights not just with hands and legs, but also with the brain, and is required to do so. To have one's kidneys get used, as in the violinist case, is no more invasive of one's person than to have to obey orders, to have to focus with one's brain and mind on the tasks that one's commander requires one to focus on. The loss of autonomy on a military campaign is, if anything, greater than in the pregnancy and violinist cases.
Note, too, that the draft argument gives support for two claims. First, it supports a moral claim: It is one's duty, on pain of freeriding, to do one's bit for saving lives. Second, it supports a policy claim: It can be both permissible and reasonable for the state to require this of one.