On June 1, 1913, two days after the signing of the Treaty of London and just 28 days before the Bulgarian attack, Greece and Serbia signed a secret defensive alliance, confirming the current demarcation line between the two occupation zones as their mutual border and concluding an alliance in case of an attack from Bulgaria or from Austria-Hungary. With this agreement, Serbia succeeded in making Greece a part of its dispute over northern Macedonia, since Greece had guaranteed Serbia's current (and disputed) occupation zone in Macedonia.^[69] In an attempt to halt the Serbo-Greek rapprochement, Bulgarian Prime Minister Geshov signed a protocol with Greece on 21 May agreeing on a permanent demarcation between their respective forces, effectively accepting Greek control over southern Macedonia. However, his later dismissal put an end to the diplomatic targeting of Serbia.

Another point of friction arose: Bulgaria's refusal to cede the fortress of Silistra to Romania. When Romania demanded its cession after the First Balkan War, Bulgaria's foreign minister offered instead some minor border changes, which excluded Silistra, and assurances for the rights of the Kutzovlachs in Macedonia. Romania threatened to occupy Bulgarian territory by force, but a Russian proposal for arbitration prevented hostilities. In the resulting Protocol of St. Petersburg of 9 May 1913, Bulgaria agreed to give up Silistra. The resulting agreement was a compromise between the Romanian demands for the city, two triangles at the Bulgaria–Romania border and the city of Balchik and the land between it and Romania and the Bulgarian refusal to accept any cession of its territory. However the fact that Russia failed to protect the territorial integrity of Bulgaria made the Bulgarians uncertain of the reliability of the expected Russian arbitration of the dispute with Serbia.^[70] The Bulgarian behavior had also a long-term impact on the Russo-Bulgarian relations. The uncompromising Bulgarian position tο review the pre-war agreement with Serbia during a second Russian initiative for arbitration between them finally led Russia to cancel its alliance with Bulgaria. Both acts made conflict with Romania and Serbia inevitable.