> It wasn’t slavery as they were paid

Being paid some amount of money doesn't magically make it not slavery. Obvious counterpoint: what if a plantation owner "paid" each slave a single modern-day penny a year, would that make it okay?

> and could be expected to be discharged at the cessation of hostilities

Being let go when you're no longer needed doesn't stop it being slavery either. Would you no longer be a slave if the plantation owner released you after the harvest season? You'd just be re-captured for the next harvest, of course - either by the same owner or a different one.

> Conscription is as old as society itself.

So is slavery. That doesn't make it okay.

Definitions of forced labour, like the ILO Forced Labour Convention of 1930 for example, have to explicitly include "by the way, it's totally okay if it is done as part of mandatory military service" clauses for a reason.

There are obviously differences between traditional slavery and conscription, but conscription is still way closer to forced labour than it is to consensual service. Just look at what happens when you try to leave, for one!