The controversial Treaty of Trianon of 1920 whereby Hungary lost one-third of its territory and population at the expense of Romania, Czechoslovakia and Yugoslavia has been the focal point of Hungarian revisionism ever since its inception. This study clarifies both the character of the treaty and the bases of the controversy by re-examining the nationalities, conflicts of the Austro-Hungarian Monarchy, the war aims of World War I, the goals and decisions of the Paris Peace Conference, the terms of the treaty and its execution.