Using a new measure called "territorial fractionalization" to indicate the extent to which traditional societies are split across current states, the authors of this study propose that fragmentation of ethnic groups and ethnonationalism is key to understanding civil conflict. They find that higher measures of territorial fractionalization significantly predict a higher risk of civil conflict, and that greater increases in territorial fractionalization also predict greater risk of civil conflict. These results hold for a global sample of data from 1945 to 2017, as well as in European and African subsamples dating back to 1886.