Even aside from records kept during the war itself, the US had a census in 1860, and then another one in 1870, and it’s probably safe to assume that most of the excess deaths in that decade, compared to the decades immediately before or after, were due to the war.
I have an ancestor who was captured at Vicksburg, paroled on condition of not fighting again, re-upped (!). All documented. And then disappeared from all records forever.
In the last year of the war, the CSA record keeping system really broke down. Desertions were a big problem. Records (old and new) got destroyed due to war. Etc.
The chaos of war is unbelievable to people who never experienced it. People move around all the time. Bodies get destroyed completely so there’s nothing to even look for. Record keeping is secondary to staying alive.
I had an uncle who was a POW in Germany.* In the winter of 1944 he was in a death march across Germany. It was beyond horrible. Those that couldn’t keep up with the march were killed. The Germans were definitely not keeping records of this.
And this is Allied POWs. Never mind Russian POWs or the peoples Germany was to trying to wipe out. While Germany is noted for it’s bureaucratic thoroughness, by the winter of '44, all bets were off.
- And yet he survived and lived into his 90s!