Conscription is a bad idea unless it is strictly necessary. But it's definitely not a contravention of the Constitution, nor is it an overreach of government. This is one of the few places where government is absolutely necessary -- to provide for the common defense. Even a libertarian would grant that.
Under Anglo-American common law, the several states may muster a portion of the militia (all able bodied males, usually between 18 and 60 years old) to active service. This is, in effect, a draft. However, it is a draft that meets Constitutional requirements and respects the limits to Federal power that was intended by the Founding Fathers. The Federal government may then call upon portions of the militias to repel invasions and suppress insurrections, as the Constitution provides.
Those conservatives who are willing to bend the Constitution in the name of foreign policy or right wing social engineering, e.g., to create better social discipline or more manly men, are hypocrites when they object to liberals using the "fearful master" of government to their ends. The ends, even if they are desirable ones, do not justify the means.