It is debatable whether one can do anything at all against one's will. In regards to conscription in the US, one could always refuse and go to jail. Also, I believe even then there was such a thing as a conscientious objector.
Even then you can refuse to cooperate. You might be shot or might be bodily dragged to jail, but you still have a choice to cooperate or not, to resist or not.
So you're saying that if you refuse to go to war, are threatened with being killed and then go to war, you are in fact going to war voluntarily? That sounds like a rather philosophical interpretation of "of your own free will".
I'm saying that I recognize that there is coercion, but also that you have a choice. Those that are ordered to do something and do it choose to do so. But they could have chosen otherwise and faced the consequences. In fact, many in the past have done just that.
It's disingenuous to say they didn't have a choice or they did it against their will, for if they really willed otherwise, they could have done otherwise (as demonstrated in the previous paragraph). They may have preferred to do something else, but not enough to actually do anything about it.
If you really and truly object to going to war or killing, then don't. You always have that choice. Don't pretend you don't.
In "On Killing" [0], the author argued that WWI troops consistently aimed over the heads of their enemies, in order to avoid ever actually killing someone.
This tendency obviously ran counter to the goals of the generals, so after WWII the military devised ways to make the soldiers comfortable with aiming at people.
That's just to say - even among active-duty soldiers, there is a spectrum of those willing to kill or not.
It's disingenuous to say they didn't have a choice
or they did it against their will
If a school bully someone tells me "give me your lunch money or I'll give you a black eye" and I give them my lunch money, would you say I've done so against my will?
I would say I had done so against my will - because although I had a choice between two bad options, I was forced to submit to a choice that was structured to deny me the option to do neither.
With conscription the third option, not going to jail and not joining the army, has been removed forcibly. If I intend to work on my farm but get conscripted I don't get the choice of staying. I will have to leave against my will.
Hey choose between joining the army or jail time! It's your free choice so it can't be against your will!
But more importantly back then American soldiers were conscripted and forced to fight against their will.