Greek – Serbian Alliance of 1913 The Greek – Serbian Alliance of 1913 was signed at Thessaloniki on 1 June 1913 , in the aftermath of the First Balkan War , when both countries wanted to preserve their gains in Macedonia from Bulgarian expansionism . The treaty formed the cornerstone of Greek - Serbian relations for a decade , remaining in force through World War I until 1924 . Background . During the First Balkan War , both Greek and Serbian armies concluded their operations in Macedonia by late 1912 , while the Bulgarian army was directed mainly towards Thrace . As a result , the former succeeded in taking control of most of the region , including its largest city , Thessaloniki , which was occupied by the Greek army in early November . Bulgaria nevertheless demanded most of Macedonia for itself , relying on the crucial role its army had played in facing the bulk of the Ottoman army in the First Balkan War , as well as on the pre - war agreements with Serbia regarding their mutual territorial claims in northern Macedonia . Both Greece , which had not signed any agreement on territorial partition , and Serbia , which had been forced by the Great Powers to cede its conquests in Albania in favour of the newly created Albanian Principality , opposed Bulgarian claims and insisted on an " uti possidetis " division of territory . Negotiations and signing of the alliance . On 9 March 1913 , the Greek Foreign Minister Lambros Koromilas instructed the Greek ambassador to Belgrade to sound out the Serbian government with a view to a bilateral alliance treaty . Preliminary discussions quickly bore fruit , and on 5 May Koromilas and the Serbian ambassador to Athens signed the first protocol , where both sides pledged mutual support against Bulgaria should the latter refuse to acknowledge the territorial status quo . The protocol also included a 50 - year commercial agreement which allowed the use of Thessaloniki by Serbian companies . This protocol was then followed by a military convention signed on 13 May at Thessaloniki . The Serbian government however refused to ratify the latter , insisting that Greece take on itself the previous Bulgarian pledge to provide 200 , 000 troops in the case of an Austrian attack . The Greek government was reluctant to agree to this , but following the clashes between Greek and Bulgarian troops among the mutual demarcation line at Nigrita and Mount Pangaion , as well as the view of Prime Minister Eleftherios Venizelos that an Austro - Serbian war would soon draw the other Great Powers into the fray , it agreed . The final Treaty of Peace , Friendship and Mutual Protection was signed at Thessaloniki on 1 June 1913 , by the Greek ambassador to Belgrade I . Alexandropoulos and the Serbian ambassador to Athens M . Boskovic . Article 1 gave a mutual guarantee of the signatories ' territorial possessions , concluded a defensive alliance and precluded each government from unilaterally concluding a peace treaty . Article 2 pledged the two states to a common front in the negotiating table against Bulgaria . Article 3 delineated the Greek - Serbian border ( presently the border of Greece and the Republic of Macedonia ) based on the demarcation line between the respective armies . Articles 4 and 5 stated each country ' s territorial claims on Bulgarian - controlled territory ( the line Gevgelija - Vardar River - Bregalnica - Osogovska Planina for the Serbs and Beles - Eleftheres Gulf for the Greeks ) , committed them to seek international mediation , and to meet Bulgarian armed aggression towards either signatory with all available forces . The attendant military convention obliged each country to come to the other ' s aid with all available forces should either be attacked by a third power , or , if one of the signatories declared war first , to maintain a favourable neutrality and conduct a partial mobilization ( 40 , 000 men for Greece and 50 , 000 for Serbia ) . The signature of the Greek - Serbian alliance marked the demise of the Balkan League . On 30 June , Bulgarian troops attacked the Greek and Serbian lines , beginning the Second Balkan War . Impact and aftermath . The alliance played an important role in World War I , when Serbia was attacked by Austria . Venizelos mobilized the Greek army , but met with the refusal of the Germanophile King Constantine I to enter the war on the side of the Entente Powers . In an attempt to force the king ' s hand , in mid - 1915 Venizelos allowed a British - French force to land in Thessaloniki in order to aid the Serbs , establishing the Salonica Front . The crisis between Prime Minister and King over the issue of the country ' s participation in the war led to the National Schism , which plagued Greek political life until the 1930s . The Treaty of Alliance was finally unilaterally cancelled by the new Kingdom of the Serbs , Croats and Slovenes in 1924 , as a reaction to the controversial Greek - Bulgarian Politis – Kalfov Protocol on minorities .