Let me begin by saying that I expect my answer to this one will be controversial, as I think there are deep feelings about this issue, and also a very broad range of considerations. So my own response does not rise above simply stating an opinion for others to consider.
For what it is worth, then: I think, as a matter of fact that within democracies military drafts should be mandatory. So, I suppose it is obvious that I think they are neither unethical nor immoral in democracies. I think in systems where the people's consent to government is not given, but simply coerced (the obvious example being dictatorships), then military conscription is almost always immoral, however.
But in democracies, I think that military drafts (universal and with only carefully conceived medical or extraordinary hardship exceptions) should be mandatory. The recent situation in which the United States finds itself gives a fairly clear ground for why I say this. It is simply far too easy for a government or regime to become involved in a war when that decision only puts at risk people who have volunteered for the military. If being in the military were instead a matter of civic duty, then the entire citizenry (including especially the families and friends of conscripts!) will be much less eager to have their countries go to war. As our fiasco in Iraq shows all too well, we should have been much more reluctant, as a nation, to get into this war--but when it was only volunteers whose lives were gone to be lost, well... It was just too easy for the rest of us (many of us with a deep sense of unease nothwithstanding) to sit idly by and just allow our government to make this decision for us. Well, the wrong decision was made, and I think such decisions would be much more difficult to make if more people in this country felt the real risks at stake.
A nation should be prepared to go to war only if and when the case for risking young lives--even those dear to us--is recognized by the majority of the citizens. I think that fewer people would have supported this foolish and reckless war if the stakes included risks to them or to their loved ones, and not just to those who, for whatever reasons--often economic, to be honest, which raises very serious equity questions--volunteered to risk their lives in the military.
The US withdrew from the Vietnam War because that war--another reckless adventure, I believe--put too many conscripts' lives at risk, and the American public finally would not put up with having their loved ones killed or maimed without adequate cause. After that War, our politicians' "wisdom" conceived the all-volunteer army--precisely because that gave them much increased capacity for military engagement without the resistance of the families and loved ones of conscripts. But this resistance is precisely what should be in place to hold in check a too-great readiness to engage in war.
Few thought the draft was immoral during either of the World Wars. That is because there were extremely good reasons to be involved in those wars. But after Vietnam and Iraq, the idea of being conscripted gives us all the creeps. It should give us the creeps! But that same reaction would impede hawkish lawmakers from expensive and deadly wars that do no credit to our country, and waste too many lives (at home and abroad) for poor or selfish reasons.
So it reduces to a simple question: Would we be so willing to vote for a candidate who wished to extend this war "as long as it takes" if it was our own son or daughter who might be the next to die or be terribly injured there? I think not, and if we went back to having the draft, we'd find our current follies ended with alacrity and conviction, and our capacity to make the same mistakes again sharply constrained.