During the prewar negotiations that resulted in the Balkan League's establishment, a secret agreement on 13 March 1912 was signed by Serbia and Bulgaria, which determined their future boundaries, effectively sharing northern Macedonia.

The Serbs responded by accusing the Bulgarians of maximalism, pointing out that if they lost both northern Albania and Vardar Macedonia, their participation in the common war would have been virtually for nothing.

When Bulgaria called upon Serbia to honour the prewar agreement over northern Macedonia, the Serbs, displeased at the Great Powers' requiring them to give up their gains in north Albania, adamantly refused to evacuate any more territory.

[12] 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 boundary between their respective forces, effectively accepting Greek control over southern Macedonia.

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.

The resulting agreement was a compromise between the Romanian demands for the city, two triangles at the Bulgaria–Romania border and 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.

The uncompromising Bulgarian position to review the prewar agreement with Serbia during a second Russian initiative for arbitration finally led Russia to cancel its alliance with Bulgaria.

[citation needed] In 1912, Bulgaria's national aspirations, as expressed by Tsar Ferdinand and the military leadership around him, exceeded the provisions of the 1878 Treaty of San Stefano, considered even then as maximalistic, since it included both Eastern and Western Thrace and all Macedonia with Thessaloniki, Edirne and Constantinople.

Even worse, the concentration on capturing Thrace and Constantinople ultimately caused the loss of most of Macedonia, including Thessaloniki, and that could not be accepted, leading the Bulgarian military leadership around Tsar Ferdinand to decide upon a war against its former allies.

On 5 February, Romania settled her differences over Transylvania with Austria-Hungary, signing a military alliance, and on 28 June, officially warned Bulgaria that it would not remain neutral in a new Balkan war.

"[17] Tsar Nicholas II of Russia was already angry with Bulgaria because the latter refused to honour its recently signed agreement with Romania over Silistra, which resulted from Russian arbitration.

The next day the government pressured the General Staff to order the army to cease hostilities, which caused confusion and loss of initiative and failed to remedy the state of undeclared war.

Bulgaria intended to defeat the Serbs and Greeks and to occupy areas as large as possible before the Great Powers interfered to end the hostilities.

At the outbreak of the First Balkan War, Bulgaria mobilized a total of 599,878 men (366,209 in the Active Army; 53,927 in the supplementing units; 53,983 in the National Militia; 94,526 from the 1912 and 1913 levies; 14,204 volunteers; 14,424 in the border guards).

To replace these casualties, Bulgaria conscripted 60,000 men between the two wars, mainly from the newly occupied areas, using 21,000 of them to form the Seres, Drama and Odrin (Edirne) independent brigades.

With the eruption of hostilities, the 8th Infantry Division (stationed in Epirus) was transferred to the front, and with the arrival of recruits, the army's strength in the Macedonian theatre increased eventually to some 145,000 men with 176 guns.

The Bulgarian 2nd army in southern Macedonia, commanded by General Ivanov, held a line from Dojran Lake southeast to Kilkis, Lachanas, Serres and then across the Pangaion Hills to the Aegean Sea.

[24] The Greek army, commanded by King Constantine I, had eight divisions and a cavalry brigade (117,861 men) with 176 artillery guns[25] in a line extending from the Gulf of Orphanos to the Gevgelija area.

[2] In a diplomatic circular that said, "Romania does not intend either to subjugate the polity nor defeat the army of Bulgaria," the Romanian government endeavoured to allay international concerns about its motives and about increased bloodshed.

[2] According to Richard Hall, "[t]he entrance of Romania into the conflict made the Bulgarian situation untenable [and t]he Romanian thrust across the Danube was the decisive military act of the Second Balkan War.

In the words of historian Richard Hall, "[t]he battlefields of eastern Thrace, where so many Bulgarian soldiers had died to win the First Balkan War, were again under Ottoman control.

[33] On 20 July, via Saint Petersburg, the Serbian Prime Minister Nikola Pašić invited a Bulgarian delegation to treat with the allies directly at Niš in Serbia.

[33] When the delegations met in Bucharest on 30 July, the Serbs were led by Pašić, the Montenegrins by Vukotić, the Greeks by Venizelos, the Romanians by Titu Maiorescu and the Bulgarians by Finance Minister Dimitur Tonchev.

"[43] Although Austria-Hungary and Russia supported Bulgaria, the influential alliance of Germany—whose Kaiser Wilhelm II was brother-in-law to the Greek king—and France secured Kavala for Greece.

Both sides made competing declarations: Savov that "Bulgaria, who defeated the Turks on all fronts, cannot end this glorious campaign with the signing of an agreement which retains none of the battlefields on which so much Bulgarian blood has been shed," and Mahmud Pasha that "[w]hat we have taken is ours.

The inhabitants were denied voting rights, ostensibly because the cultural level was considered too low, in reality, to keep the non-Serbs, who made up the majority in many areas, out of national politics.

In October and November 1913, British vice-consuls reported systematic intimidation, arbitrary detentions, beatings, rapes, village burnings and massacres by Serbs in the annexed areas.

At the strong insistence of Austria-Hungary and Italy, both hoping to control for themselves the state and thus the Otranto Straits in Adriatic, Albania acquired officially its independence according to the terms of the Treaty of London.

With the delineation of the exact boundaries of the new state under the Protocol of Florence (17 December 1913), the Serbs lost their outlet to the Adriatic and the Greeks in the region of Northern Epirus (Southern Albania).