The collapse of an empire can result in the division of families and the redrawing of geographical boundaries. New leaders promise the return of people and territories that may have been lost in the past, often advocating aggressive foreign policies that can result in costly and devastating wars. The final years of the Austro-Hungarian and Ottoman empires, the end of European colonization in Africa and Asia, and the demise of the Soviet Union were all accompanied by war and atrocity.

These efforts to reunite lost kin are known as irredentism--territorial claims based on shared ethnic ties...