Faded ink. Inconsistencies. Fires. The deadliest conflict in American history, the Civil War, also had some of its worst-kept records, making the true number of casualties seem forever elusive.

For decades, historians have cobbled together clues. But thanks to a newly released set of census records spanning three decades, researchers have landed on a firmer estimate of lives lost: 698,000.

The analysis, published this week in The Proceedings of the National Academy of Sciences, also suggested that the Confederate states fared much worse than the Union, with a mortality rate more than twice as high. The granular nature of the census data means that researchers who build upon the work will be able to better understand the long-term impact of the war in the hardest-hit regions.

The team of researchers thought it would start by looking at the loss in sheer numbers. The problem was that the data set didn’t seem to exist. Then just over a decade ago, a demographic historian developed a new method of calculating the loss: using census data.

— The New York Times