The war between Italy and the Ottoman Empire was still going on, and the Balkan countries were also ready to move. Among them, Bulgaria and Serbia were the first to reach an agreement, and Serbia faced Austria-Hungary pressure. At that time, however, Serbia was very interested in an alliance with Bulgaria.

The abrupt annexation of Bosnia and Herzegovina by Austria-Hungary in 1908 shocked the Balkan states and accelerated their approach and alliance. Faced with great pressure from Austria-Hungary, Serbia was keen to receive Bulgaria's support. Although King Ferdinand of Bulgaria was the son of Prince Coburg Koda of the German royal family, and had a strong hatred of Russia and Austria, he also recognized the need for an alliance with Serbia because the two countries had a common interest in opposing external threats.

However, the alliance between Bulgaria and Serbia was not all smooth sailing, because the goals of the alliance between the two countries are not the same, and it can be said that each has its own intentions. Bulgaria saw the Ottoman Empire as its number one enemy and wanted Serbia to help it gain Macedonia and Thrace. Serbia, on the other hand, saw Austria-Hungary as the greatest threat and wanted to get involved in Macedonia, so the two sides did not get along.

In November 1909, King Ferdinand made a special trip to Serbia, which did not yield positive results. Upon hearing this, Austria-Hungary intervened and tried to sabotage it, warning the Bulgarian king not to expect assistance from Serbia in a future war. Austria-Hungary could help Bulgaria, and promised that if the Ottomans attacked Bulgaria, Vienna would not sit idly by, and would assist Bulgaria in carving up Macedonia with Serbia. Kaiser Wilhelm II was also opposed to the alliance of the Balkan states, which caused great trouble for the proximity of Bulgaria and Serbia.

At that time, Serbia was very interested in an alliance with Bulgaria, which had a great influence on the domestic situation, and many militant groups at that time demanded that the government form an alliance with other Slavic countries as soon as possible in order to jointly deal with Austria-Hungary and the Eisman Empire. In addition, for the sake of foreign policy, Serbia is trying to consolidate its position in the Balkans and to that end it is trying to expand its sphere of influence. According to Serbian Foreign Minister Milanovic, if it is not united with Bulgaria, it will no longer be possible to exert influence on Croatia and Slovenia.

As a result of the changes in the Bulgarian government, last year the People's Party led by Geishov and the pro-Russian Progressive Party headed by Danev came to power to form a coalition government.

In April last year, the leader of the Serbian Radical Party, Pasić, stressed in a conversation with the Bulgarian ambassador to Serbia, Tosev. Once the two countries were united, the aim was to oppose the common enemy on the one hand, and at the same time to expand the territory of the two countries by dividing up the Ottoman Empire in Europe, and to set the tone for the alliance between Serbia and Bulgaria at one time.

The Ottoman territory to be partitioned by Pasić is clearly Macedonia, and because of the mix of Serbs, Greeks, Turks, Bulgarians, Albanians, Romanians, etc., Serbia, Bulgaria, and Greece all have their own claims to this territory. Hopefully, if not fully controlled, then at least some of it will be obtained. Pasic from the Bulgarian side agreed, after which negotiations between the two sides were triumphantly accelerated.

The attitude of the European powers, especially Russia, played a pivotal role in the proximity of Bulgaria and Serbia. Russia was in favour of an alliance between Bulgaria and Serbia and had been in extensive contact with Bulgaria since 1910. Bulgaria is eager to regain its ambitions of "Greater Bulgaria" and wants to get the support of Russia, so it is also actively drawing closer to Russia. However, in the course of the negotiations, the two sides discovered that there were significant differences in their strategic objectives.

The first was that Bulgaria had high expectations and hoped to take Salonika and Adrianople (the best areas of Macedonia that were intended to be seized with Russian help). Russia, on the other hand, feared that it would take too much of its obligations to Bulgaria and involve itself in conflict with the other European powers, which was the last thing it wanted. The second was Russia's insistence that the goal of the alliance between Bulgaria and Serviyang should be directed against Austria-Hungary and should not be an enemy of Turkey, which of course Bulgaria rejected. As a result, negotiations with Bulgaria have reached an impasse.

But since last year, Russia's attitude towards Bulgaria has become enthusiastic again. Efforts were made to bring Serbia closer to him, and to emphasize that Russia could consider an alliance with Bulgaria only on the condition of an alliance between the two countries, thus injecting a boost into the negotiations between Serbia and Bulgaria.

Not only that, the Russian ambassador to Serbia, Hartwig, and the Bulgarian ambassador, Nelyudov, also began to take an active part in the diplomatic negotiations between the two sides, and proposed a mutually acceptable plan for the most stinging Mariton question, in which the northern, southeastern corners of Macedonia would be given to Bulgaria, and the northwestern corner would be given to Serbia, and the ownership of a piece of land (including Skopje) between the two would be decided by the Russian Tsar in the future. (The southern part of Macedonia was left to Greece because it needed the Greek navy to block the reinforcements of troops from the Ottoman territories and to cut off its most important sea transport.) ）

The settlement of the Macedonian problem has removed one of the greatest obstacles to the alliance between the two countries.

As progress accelerated, on March 12, 1912, Bulgaria and Serbia formally signed a treaty of friendship and alliance, which included military obligations to each other in the event of an attack on one of the two countries. It was also stipulated that Ottoman Turkish rule in Kosovo and Macedonia should be ended, and that a secret agreement should be reached on the partition of Macedonia.

The military provisions attached to the treaty were as follows: in the event of war, Bulgaria should provide 200,000 troops, Serbia should provide 150,000 troops to fight together, of which 100,000 should be sent to Macedonia to help Bulgaria. The Russian Government was very satisfied with this treaty, and Foreign Minister Sasonov could not help but say with joy when he heard the news. "Good! It couldn't be better! Now there are 500,000 bayonets to defend the Balkans - it will forever block the German infiltration and the Austro-Hungarian invasion! ”

In addition to actively seeking a treaty with Serbia, Bulgaria also sought an alliance with Greece. In 1910, when Viniziros became Prime Minister of Greece, he showed a keen interest in Bulgaria, and a committee of 10 prominent politicians was set up in the city to study how to improve relations between the two countries, and initially many concrete proposals were made.

In 1911, Bulgaria and Greece negotiated the Macedonian question, which did not lead to an agreement, but increased the understanding between the two countries. After the signing of the Serbian-Bulgarian treaty, as well as Italy's threat to shell the Dardanelles, Greece took a more active approach.

In April 1912, Viniziros proposed an alliance between Greece and Bulgaria, and then at the request of the Bulgarian side, a positive response to the Macedonian question was made, which was run by Greek Prime Minister Viniziros, and negotiations between Bulgaria and Greece were conducted in top secrecy without the knowledge of the Greek government.

Finally, on May 29, 1912, the Bulgarian Treaty of Defense League was signed, stipulating that Bulgaria should provide 300,000 troops and Greece 120,000 troops in the war against Turkey.

Bulgaria, having formed an alliance with Serbia and Greece, finally set its sights on Romania behind them, while Eder had been waiting for them to come to the door.