When does killing in a time of war become a genocide — the deliberate attempt to wipe out some, or all, of a national, ethnic, religious or racial group? There is universal agreement that the Nazi attempt to annihilate the Jews and the Roma people during the Second World War constituted genocide, but parsing the differences between genocide and other crimes against humanity is complex. A Genocide Convention exists, and was one of the founding documents of the United Nations in 1948. Yet applying the laws against genocide since that time has been a struggle. Canadian scholar William Schabas wrote one of the first and most important books on the subject.