If the people of a country are in serious danger of being enslaved by an enemy, chances are that many people will voluntarily join the army. An unpopular war of choice, on the other hand, may struggle greatly to raise as many voluntary troops as the government desires, and this is a circumstance where conscription is both most likely and most questionable. In WW2, British men not only "signed up" but were conscripted, but in this case I think you rightly allude to the fact that it was called for under the circumstances; there was a real danger of much greater evils than conscription being visited on the British people if the Nazis were to triumph. But since the thread started with a question about Vietnam, I think it's worth considering the smallness of the possibility of the Vietnamese achieving the enslavement of the American people or anything of the kind at the point in time when conscription was instituted for the prosecution of that war.