The conviction at the Hague this week of Serb leader Radovan Karadzic deserved more attention than it has received. The Karadzic conviction was no mere tidying up of the Balkan wars of the 1990s. It is relevant to the brutal events in Brussels that transfixed the world this week, as have similar bloodlettings at the hands of Islamic State.

Karadzic was a Serbian demagogue who drove the first organized genocide on European soil since World War II. The details of his conviction, including the killing of 8,000 Muslim males in...