The League of the Balkans[a] was a quadruple alliance formed by a series of bilateral treaties concluded in 1912 between the Eastern Orthodox kingdoms of Greece, Bulgaria, Serbia and Montenegro, and directed against the Ottoman Empire,[1] which still controlled much of Southeastern Europe.
The Balkans had been in a state of turmoil since the early 1900s, with years of guerrilla warfare in Macedonia followed by the Young Turk Revolution, the protracted Bosnian Crisis, and several Albanian Uprisings.
In consequence, it started engineering an ambitious plan for indirect expansion through the creation of friendly and closely allied states under Russian patronage in the Balkan Peninsula.
With the antagonism of the European powers mounting, and smarting from her humiliation by the Austrians at the Bosnian Crisis, Russia sought to gain the upper hand by creating a Russophile "Slavic block" in the Balkans directed against Austria-Hungary and the Ottomans.
Thus, the final agreement between the two countries stipulated that in the event of a victorious war against the Ottomans, Bulgaria would receive all of Macedonia south of the Kriva Palanka–Ohrid line.
After the successful coup d'état for the incorporation of Eastern Rumelia, Bulgaria had orchestrated a methodical scenario of indirect expansion through the creation, in the multi-ethnic Ottoman-held Macedonia (for many centuries an administrative rather than a nationalistic name), of a united, liberating and revolutionary organization, the IMRO, allegedly without national color.
After initial success, Serbia and especially Greece realized the true purpose of IMRO and consequently, a vicious guerrilla war, the so-called Macedonian Struggle broke out between Bulgarian and Greek-backed armed groups within Ottoman Macedonia.
Bulgaria then turned to the more direct method of expansion through winning a war, building a large army for that purpose, and started to see itself as the "Prussia of the Balkans".
In the discussions that led Greece to join the League, Bulgaria refused to commit to any agreement on the distribution of territorial gains unlike the deal with Serbia over Vardar Macedonia.
In addition, the Italian occupation of the Greek-inhabited Dodecanese Islands served as a warning for Greece of the consequences of staying out of a future war against the Ottomans.
Mounting tensions effectively tore the League apart, and the Second Balkan War broke out when Bulgaria, confident of a quick victory, attacked its former allies Serbia and Greece.
That eventually led to the National Schism, which greatly contributed to the loss of the next war against Kemalist Turkey in Asia Minor, and dominated Greek politics for over a half of a century.
The outcome of the Balkan Wars caused a permanent break-up of the Russo-Bulgarian alliance and left Serbia and Montenegro as the only allies of Russia in the critical region.