This is where the deductive syllogism comes in handy. All men are mortal. Therefore, Socrates is mortal. When the conclusion follows necessarily from the premises in this way, the argument is said to be valid. Moral arguments can be analyzed in a similar fashion.
Military conscription is a form of involuntary servitude.
Therefore, military conscription is immoral. Notice the difference between the major premise and the minor premise. The conclusion follows necessarily from the premises. In other words, if the premises are true, then the conclusion must be true as well. Let us consider each of these in turn.