Treaty of Neuilly - sur - Seine The Treaty of Neuilly - sur - Seine required Bulgaria to cede various territories , after Bulgaria had been one of the Central Powers defeated in World War I . The treaty was signed on 27 November 1919 at Neuilly - sur - Seine , France . The treaty required Bulgaria The signing ceremony was held in Neuilly ' s town hall ( hôtel de ville ) . In Bulgaria , the results of the treaty are popularly known as the Second National Catastrophe . Bulgaria subsequently regained South Dobruja as a result of the Treaty of Craiova . During World War II , together with Nazi Germany , it temporarily reoccupied most of the other territories ceded under the treaty . Territories ceded to the Kingdom of Serbs , Croats and Slovenes . Four minor regions ( historiographically referred to by Bulgarians as the Western Outlands ) had been part of Bulgaria from its inception as a principality in 1878 , except for the region around Strumitsa , which became part of Bulgaria in 1912 . Bulgaria was internationally recognised as an independent country in 1908 and controlled these territories until 1919 when they were ceded to the Kingdom of the Serbs , Croats and Slovenes under the Treaty of Neuilly . The cession of the region was partly a compensation for the occupation of the southern and eastern part of Serbia by Bulgarian troops between 1915 and 1918 , and was partly motivated by strategic reasons . The old political boundary between Bulgaria and Serbia followed a chain of high mountain ridges , whereas the new one gave significant military and strategic advantages to the Serbs : it dangerously exposed the Bulgarian capital of Sofia and significantly reduced the military threat to eastern Serbia in case of a Bulgarian invasion ( see also Balkan Wars and World War I ) . The Treaty of Neuilly was one of the series of treaties after World War I , which included the Treaty of Versailles , the Treaty of Saint - Germain , the Treaty of Trianon , and the Treaty of Sèvres , which were intended to diminish the military and political strength of the defeated members of the Central Powers . Some areas with a Bulgarian majority population ( such as Bosilegrad and part of the Dimitrovgrad municipality ) were ceded to Serbia ( part of the Kingdom of Serbs , Croats and Slovenes ) . Bulgarian sources claim that the treaty was intended to last for twenty years and that all the ceded territories should have been returned to Bulgaria in 1939 , but that was not specified in the treaty . Area and population . Territories ceded by the treaty to the then Kingdom of Serbs , Croats and Slovenes cover an area of in what is now Serbia and in what is now the Republic of Macedonia . In Serbia , to which the term generally applies in Bulgaria , the territory ceded is split between the modern Serbian District of Pirot ( municipality of Dimitrovgrad and smaller parts of the municipalities of Pirot and Babušnica ) and District of Pčinja ( municipality of Bosilegrad and a small part of the municipality of Surdulica ) . It also includes a small section along the Timok River in the municipality and District of Zaječar , composed by eight localities ( seven populated by Vlachs and one populated by Bulgarians ) . In 1919 , the area corresponded to the following parts of the Bulgarian " okrug s " : Kyustendil , , Tzaribrod , Tran , Kula and Vidin . Bulgarian sources claim that the Bulgarian population made 98 % of the population in Bosilegrad and 95 % of the population in Tzaribrod at the time . In the Yugoslav census of 1931 , all South Slavs were simply counted as Yugoslavs ( Serbs , Croats , Slovenes , Bulgarians ) so a comparison could not be made . According to the last Census in Serbia from 2002 , Bulgarians made 50 % and 71 % of population in Dimitrovgrad and Bosilegrad respectively .