It is in the relations of Bulgaria and Serbia, however, that the key to the Balkan situation is to be found. The Serbo-Bulgarian treaty of February 1912, which formed the groundwork of the Balkan alliance, had limited Serbia's sphere of influence to northern Macedonia and referred to the arbitration of the Russian Tsar any disputes arising from conquests to the south of a certain specified line. Serbia was tacitly given a free hand in her attempt to reach the sea in Northern Albania. The action of Austria-Hungary in vetoing her access to the Adriatic forced Serbia to turn her eyes from west to south and to seek her economic outlet to the sea down the valley of the Vardar to Salonica and the Aegean. The cession of Monastir, Ochrida, and the Vardar Valley to Bulgaria would have rendered this impossible, for it would not merely have driven a wedge between Serbia and Greece, but would have placed two customs frontiers, the Bulgarian and the Greek, between Serbia and the sea, instead of only one, the Turkish, as hitherto. Shut in upon all sides, with all hope of expansion blocked by the powerful Dual Monarchy to north and west and by a big Bulgaria to east and south, Serbia would have found herself in a worse position than before the war. The Bulgarians, intoxicated by their victories over the Turks and seduced by the promptings of the Austrian tempter, turned a deaf ear to the arguments of their Serbian allies, and insisted upon their pound of flesh. They failed to realise that the most effective way of inducing the Serbs to evacuate Macedonia was to give them adequate backing in their demand for an Adriatic port. Every fresh intrigue of Sofia with Vienna confirmed Belgrade in its view of the vital necessity for retaining the Vardar Valley. The hoary argument that "circumstances alter cases," appeared anew in the garb of the Bismarckian theory that all treaties are subject to the provision "_rebus sic stantibus_"--a theory which many great international lawyers have unhesitatingly endorsed. In this form it appealed as irresistibly to the Serbs as did the rival shibboleth of "The treaty, the whole treaty, and nothing but the treaty" to the Bulgarians. It is impossible to absolve either side from blame; for the Serbs, in formally denouncing a treaty into which they had voluntarily entered, were doing exactly what they had so bitterly resented in Austria-Hungary's treatment of Bosnia, while the Bulgarians, in flouting the Tsar whom they had named as arbiter and in attempting to uphold the treaty by brute force and treachery, abandoned the ground of law, and placed themselves openly in the wrong.