The first draft of the Convention included political killings, but these provisions were removed in a political and diplomatic compromise following objections from some countries, including the USSR, a permanent security council member. The USSR argued that the Convention's definition should follow the etymology of the term, and may have feared greater international scrutiny of <answer>its own Great Purge</answer>. Other nations feared that including political groups in the definition would invite international intervention in domestic politics. However leading genocide scholar William Schabas states: “Rigorous examination of the travaux fails to confirm a popular impression in the literature that the opposition to inclusion of political genocide was some Soviet machination. The Soviet views were also shared by a number of other States for whom it is difficult to establish any geographic or social common denominator: Lebanon, Sweden, Brazil, Peru, Venezuela, the Philippines, the Dominican Republic, Iran, Egypt, Belgium, and Uruguay. The exclusion of political groups was in fact originally promoted by a non-governmental organization, the World Jewish Congress, and it corresponded to Raphael Lemkin’s vision of the nature of the crime of genocide.” 
What atrocity motivated a self-serving USSR to object to the provision of political killings drafted into the Convention?
its own Great Purge