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.

The events of the great war have already modified the problem.  The one unanswerable argument of the Serbs in declining to surrender Macedonia was the plea that they would then have nothing to offer Bulgaria for her neutrality or her support when their own inevitable day of reckoning with Austria should arrive.  In short, Veles, Monastir and Ochrida were widely regarded as a pledge to be held until Bosnia and Dalmatia could be redeemed, but then to be handed over to the Bulgarians.  It is true that the Serbo-Bulgar War of 1913 and the passions which it aroused have converted this feeling into one of reluctance to sacrifice what was bought at such a fearful price.  But the moment has now arrived to translate an instinct into a reality.  If Southern Slav Unity is to be achieved, a binding promise, under the guarantee of the Entente Powers, must be given to Bulgaria, that, in proportion as the work of Serbo-Croat unification is achieved, the Macedonian frontier will be revised in favour of Bulgaria.  It is possible that Bulgaria may prefer a different formula, according to which the Tsar with the approval of his Western Allies should arbitrate upon the original Serbo-Bulgar treaty.  Any such concession to Bulgarian sentiment ought not to be resented in Serbia, in view of the great issues involved.  It is obvious that Serbia cannot hope to achieve her national unity unless Bulgaria abstains from hostile action, or to consolidate her new position when won unless she can win Bulgaria’s active friendship.  The latter by her intervention could at any moment turn the scales against Turkey or against Serbia, and it is thus essential that the Allies should treat her now with a generosity proportionate to the callous neglect with which Europe left her to her fate in September 1913.