Just to clarify: the US Civil War pre-dates the Geneva Conventions. So Confederate soldiers weren’t confused about whether the Conventions applied.

But they sent letters home wondering why they hadn’t been informed of charges against them, given a bail hearing, or promised a speedy trial. I think it’s a safe bet they also didn’t have an attorney represent them during any questioning. They were confused about why they were in a prison even though they hadn’t been arrested for a crime.