Between 1894 and 1924, the number of Christians in Asia Minor fell from some 3-4 million to just tens of thousands—from 20% of the area’s population to under 2%. Turkey has long attributed this decline to wars and the general chaos of the period, which claimed many Muslim lives as well. But the descendants of Turkey’s Christians, many of them dispersed around the world since the 1920s, maintain that the Turks murdered about half of their forebears and expelled the rest.

The Christians are correct. Our research verifies their...