http://www.economist.com/blogs/economist-explains/2015/04/economist-explains-21?zid=312&ah=da4ed4425e74339883d473adf5773841 "In 1948 the United Nations adopted a convention aimed at preventing and punishing genocide, which it defined as the “deliberate and systematic destruction, in whole or in part, of an ethnical, racial, religious or national group”. Getting agreement on the text involved compromises. Targeting victims because of their class, for example, was not classed as genocide: Stalin would hardly have signed if it meant being held to account for his mass slaughters of “middle peasants” and the like."