Contradictions between the regions of Belgium - Wallonia and Flanders - have a long history associated with uneven political, economic and cultural development of the territories, which in 1830 became parts of an independent state, the Kingdom of Belgium. The "Letter to the King", written by eminent Belgian politician and writer, socialist Jules Destree (1863 - 1936 gg.), is one of the most interesting sources on the history of Belgium of this period, It became a kind of manifesto of balance between the ideas of separatism and unitarianism. This article includes excerpts from the "Letter to the King", which were given for the first time in the author's Russian translation and conducted historical criticism of the source. The study is based on a set of scientific methods and approaches, including the principle of scientific objectivity and systematic approach, used in historical research. The main methods are problematic and historical-comparative analysis, classification and comparison of political and historical concepts. An external source of criticism included information about the place and the time of the creation of document, the biography of the author studies; internal critic source recreates the idea of the letter and the background of historical events. Prominent Belgian politician and writer, socialist Jules Destree (1863 - 1936), highly appreciated the personal qualities of King Albert I, appealed to his experience and political vision, describing all the problems Walloons faced in their opposition to the Flemish. Destree in his letter opposed the unequal development of the regions of Belgium and the protectionist policies of the central government in respect to the detriment of Flanders Wallonia. He summarized and formulated the factors that, in his opinion, interfere with the full development of his country. Destree pays great attention to linguistic differences, which were at the heart of conflicts between Walloons and Flemings, and are still shaken by Belgium. He reflects on the fact that the ...