The enemy of my enemy is my friend (sort of)
Greek poster celebrating the Balkan League
The construction of the Balkan League was a complex and in many ways a precarious process. It was no easy thing to put together a complete network of alliances, since the ambitions of Greece, Bulgaria, Serbia, and Romania (not to mention such smaller fry as Montenegro and Albania, the latter not yet a country) tended to be incompatible. In the event, Greece, Bulgaria, Serbia, and Montenegro were the only countries that joined the League. This Greek propaganda poster skips over the difficulties of putting it together and goes directly to the war, which it pictures as a more perfectly collaborative effort than it actually was.
The outstanding example of this incompatibility was Macedonia (which from Roman times until the 19th century had been a geographical rather than a political entity). Its several nationalities couldn’t be neatly separated by any kind of border. In the southernmost part, along the Aegean coast, the largest population group was Greek, or so Greece claimed; north of there it was mostly a community of Slavs whom the Serbs insisted were Serbs and the Bulgarians insisted were Bulgarians. Their language was not the same as that spoken by either Slavic neighbor. Bulgaria could claim accurately that the Macedonian tongue was much closer to Bulgarian than it was to Serbian, but tended to claim instead that it actually was Bulgarian. Mixed in with the Slavic Macedonians, were authentic Serbs, authentic Bulgarians, Turkish colonists, a few Greeks, and a whole lot of Albanians. All of these peoples were distributed over the landscape in a rich profusion that defied the skills of ethnic mapmakers. French cooks had been inspired to adopt the name of the region (Macédoine in French) for a mixed fruit salad.

Venizélos and the leaders of the other interested Balkan countries applied their diplomatic skills to the effort of bundling these various, and sometimes mutually contradictory, national causes into an alliance that would be capable of winning a war against the Ottomans. Each country knew that the Turks must be driven out by force if its own national cause was to have any hope of success. Tsarist Russia also encouraged the project, for its own purposes.
Balkan aspirations—what they wanted
This map attempts to summarize the territorial ambitions of the Balkan nations on the eve of the Balkan wars. In all cases, their primary motive was to liberate and unite with their “unredeemed brethren” who suffered under foreign rule—but all were capable of self-deception in such matters as just how many residents of the territory they coveted were in fact members of their own ethnic group, and how seriously the non-members in the same territory might object to being included.

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In the negotiations that created the Balkan League, Greece, Serbia, and Montenegro were able to reach agreements that balanced their ambitions for expansion. The ethnic facts on the ground made mutual accommodation comparatively easy for these three countries.

Russia, the only Orthodox Christian great power, had influence on both Serbia and Bulgaria, which were also Orthodox Christian—but it was fonder of Bulgaria. This was due more to geography than politics: Bulgaria, unlike Serbia, had a border with the Ottoman Empire, and this border was conveniently near Constantinople, which figured in Russia’s plans for future conquest. That made Bulgaria more useful as an ally than Serbia could be. Serbia knew that, but also appreciated Russia as a power opposed to both the Ottoman and Austro-Hungarian empires, both of which were obstacles to Serbian ambition and threats to its well-being. Through friendly pressure, the Russians got the two small countries to work out and sign an agreement on how they’d deal with the territory they hoped to conquer.

The arrangement they agreed on was less than perfectly coherent, however. Bulgaria recognized Serbia’s interest in Kosovo (the center of its 14th-century empire) and its desire to expand westward toward the Adriatic. Serbia recognized Bulgaria’s interest in Thrace, the territory that extended eastward from Macedonia to Constantinople. These recognitions amounted to pledges of non-interference rather than mutual assistance. Both armies, it was decided, would occupy Macedonia, and both countries would argue, in the ensuing peace negotiations, for Macedonian independence. Should this attempt fail, they would divide the territory between themselves. Southern Macedonia was earmarked for Bulgaria; northern Macedonia (including Skopje, its second largest city and present-day capital) was designated a “disputed zone,” and it was agreed that, if Serbia and Bulgaria could not reach a postwar agreement on where to draw the boundary, they would accept arbitration by the Russian tsar.

This agreement made the Bulgarians happy. They felt certain that independence for Macedonia would be a mere prelude to its becoming part of Bulgaria—and if independence wasn’t achieved, their friend and protector Tsar Nicholas would see that their claim to Macedonian territory was honored.

Greece had designs on southern Macedonia, which contained not only a substantial Greek population, but also Macedonia’s largest city, the major seaport of Salonika. However, Venizélos found the Bulgarians unwilling to make a deal with him. This was due less to anti-Greek antagonism than to frustrated ambitions going back to the Russo-Turkish war of 1877.
Bulgaria as revised by the Congress of Berlin
The initial peace treaty (San Stefano, 1877) had given Bulgaria, Russia’s ally and protégé, nearly all of Macedonia and quite a bit of Thrace. But the European powers intervened, called a high-level Congress, and replaced the Treaty of San Stefano with the Treaty of Berlin (1878). This map shows how drastically that treaty redesigned Bulgaria. It was reduced almost to its prewar size (solid red on the map), though it was granted a kind of semi-sovereignty over the northern part of ancient Thrace, where the population was strongly Bulgarian. This region was designated “Eastern Rumelia,” based on the Turkish name for a larger district that it was part of, and within a few years, after a bloodless revolution, it came under full Bulgarian control. (It’s shown with diagonal red and orange stripes.)

That gain, however, was far from enough to satisfy the Bulgarians, who still had no part of Macedonia and no outlet to the Aegean Sea. The red and gray striped area shows the parts of Macedonia (west) and Thrace (east) that Bulgaria had briefly possessed until the Congress of Berlin gave them back to the Ottomans. Bulgaria still felt entitled to this territory. Note that a small part of its 1877 gains had been handed to Serbia (red and purple stripes); this loss was apparently accepted as part of the Balkan League negotiations.

By 1912, the Bulgarians had, with German help, built a large, well equipped and well trained army that they were confident could easily dominate the Greeks, who had performed so miserably in their 1897 war against the Turks. This perceived military advantage was an important part of Bulgarian planning—they reasoned that their armies should be able to roll over all the territory they intended to annex without encountering any effective challenge from Greece.
Greek fleet in its home port of Fáliro, October 1912
Their estimate of the Greek army was a bit out of date, but—like every state that took part in the negotiations—the Bulgarians knew full well that Greece was the only member of the proposed league that owned an adult-sized navy. Whatever their army might lack on the ground, their fleet, it was generally acknowledged, would be able to prevent the Ottomans from moving troops to their European territory, except over land from Constantinople, a long distance from where most of the fighting would take place. Dimítrios Pánas, Greece’s ambassador in Sofia, argued that Greece would, in effect, contribute 600,000 soldiers to the war effort: “We can place an army of 200,000 men in the field and then our fleet will stop about 400,000 men being landed by Turkey upon the southern coast of Thrace and Macedonia, between Salonica and Gallipoli.” [Quoted by Richard C. Hall in The Balkan Wars 1912–1913.]

That naval contribution was a necessary condition for victory. So Bulgaria and Greece signed a pact that postponed all discussion of how the spoils should be divided until the war was over.

C. M. Woodhouse (in Modern Greece: A Short History) gives this summary of the negotiations that constructed the Balkan League:
The Balkan League of 1912 was a remarkable but precarious achievement. It linked four countries—Greece, Serbia, Montenegro and Bulgaria—in a network of bilateral agreements, not all of them formulated on paper. Serbia and Bulgaria reached agreement on their conflicting claims through the good offices of Russia. Montenegro entered into written conventions with both Serbia and Bulgaria. Greece had unwritten agreements with both Serbia and Montenegro, which caused no difficulty because their claims did not conflict. The last link, which was an understanding between Greece and Bulgaria, presented the greatest difficulty because of the inextricable conflict of their claims in Macedonia. Venizélos persuaded the Bulgarian government that the only solution was to postpone discussion of the spoils until after the victory was won. On this basis the alliance was completed. There were only two omissions from it: the Romanians, who had unacceptable claims against both Bulgaria and Turkey; and the Albanians, who were not yet recognized as a nation, though the Austrian government at least was determined that they should be. Both were to make themselves felt before the Balkan Wars were over.