I admire your sense of duty, but want to play devil's advocate for a second.
Should a slave continue to be enslaved out of a sense of duty to their master? Should a kid brought up in a religious family choose to remain in that faith merely out of a sense of duty?
It's curious how patriotism seems to instill a sense of duty in rational minds where even religion can find it tough to stick.
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.
I was comparing patriotism to slavery, not the draft per-se. The concept of feeling duty for "one's country" seems very weird when you look at it with some detachment. In a very literal sense, my loyalties end at my family, let alone my town, county, country, planet, or whatever.
In any case, modern wars are far less clear. In terms of this specific post, no-one from Vietnam was getting ready to invade or enslave the US or Europe. It could be argued Vietnam was a necessary show of force to demonstrate that the West wouldn't allow the USSR to mark its mark all around the world, but it'd be a tenuous argument at best.
Should a slave continue to be enslaved out of a sense of duty to their master? Should a kid brought up in a religious family choose to remain in that faith merely out of a sense of duty?
It's curious how patriotism seems to instill a sense of duty in rational minds where even religion can find it tough to stick.