The Kingdom of Bulgaria participated in World War I on the side of the Central Powers from 14 October 1915, when the country declared war on Serbia, until 30 September 1918, when the Armistice of Salonica came into effect.
After the Balkan Wars of 1912 and 1913, Bulgaria was diplomatically isolated, surrounded by hostile neighbors while lacking the support of any great power. Negative sentiment grew particularly in France and Russia, whose officials blamed Bulgaria for the dissolution of the Balkan League, an alliance of Balkan states directed against the Ottoman Empire. Bulgaria's defeat in the Second Balkan War in 1913 turned revanchism into a foreign policy focus.
When the First World War started in July 1914, Bulgaria, still recovering from the economic and demographic damage of the Balkan Wars, declared neutrality. Its strategic location and strong military establishment made the country a desirable ally for both warring coalitions, but its regional territorial aspirations were difficult to satisfy because they included claims against four Balkan countries. As the war progressed, the Central Powers of Austria-Hungary and the German Empire were in a better position to meet these demands. Bulgaria entered the war on the side of the Central Powers, invading Serbia in October 1915.
Although the smallest of the Central Powers, Bulgaria made vital contributions to their common war effort. Its entry heralded the defeat of Serbia, thwarted the goals of Romania, and catalyzed the Ottoman war effort by providing a land and rail link from Germany to Istanbul, that is, on Via Militaris.
Though the Balkan theatre saw successful campaigns of rapid movement by the Central Powers in 1915 and 1916, the conflict degraded into attritional trench warfare on both the Northern and the Southern Bulgarian Fronts after most Bulgarian goals were satisfied.Crampton, pg.139 This period of the war further damaged the economy, creating supply problems and reducing the health and morale of Bulgarian troops. Despite achieving national territorial aspirations, Bulgaria was unable to exit what otherwise would have been a successful war, weakening its will to continue to fight. These stresses intensified with time, and in September 1918, the multinational Allied armies based in Greece broke through on the Macedonian Front during the Vardar Offensive. Part of the Bulgarian Army quickly collapsed, and open mutiny followed as rebellious troops proclaimed a republic at Radomir.Tucker (1996), p. 151. Forced to seek peace, Bulgaria requested an armistice with the Allies on 24 September 1918, accepting it five days later. For the second time in only five years, Bulgaria faced national catastrophe. Tsar Ferdinand I assumed responsibility, abdicating in favor of his son Boris III on 3 October.Crampton, pg.143
The 1919 Treaty of Neuilly formally concluded Bulgaria's participation in World War I. Stipulations included the return of all conquered territories, the cession of additional territories and the payment of heavy war reparations.
Background
Bulgaria in the First Balkan War
When Bulgaria proclaimed its independence from the Ottoman Empire on 22 September 1908, its status was promoted to that of a kingdom and Prince Ferdinand of Bulgaria assumed the title of Tsar. The country was now able to focus on completing its national unification by turning its attention toward the lands populated by Bulgarians that remained under Ottoman control.
To achieve its goals, the Bulgarian government, under Prime Minister Ivan Geshov, approached the governments of the other Balkan countries in hopes of creating an alliance directed against the Ottomans. His efforts culminated in a series of bilateral treaties concluded in 1912 to form the Balkan League. By summer of the same year, Ottoman grip on their Balkan provinces deteriorated rapidly in Albania and Macedonia, where open rebellions had erupted.Crampton, pg.132Hall, The Balkan Wars... pg.132 The Allies decided to exploit the vulnerable state of the Ottoman Empire and declared war on it in October 1912.
The opening stages of the First Balkan War began with decisive Allied victories in both Thrace and Macedonia. Within a month, the Ottomans found themselves driven back by the Bulgarians to within 40 kilometers of Constantinople and badly beaten by the Serbians and the Greeks.Hall, The Balkan Wars... pp.43,66–67 A short armistice brought no conclusion to the conflict and fighting once again broke out in January 1913. A major Ottoman counter-offensive was defeated by the Bulgarians, who also seized the fortress of Adrianople in March and finally forced the Ottoman Empire to admit defeat and return to the peace table. While the Bulgarian army was still fighting, a new challenge arose from the north: Romania demanded territorial compensations from Bulgaria in return for its neutrality during the war.Crampton, pg.133 A protocol, signed after several conferences held in Saint Petersburg, sought to resolve the dispute by rewarding Romania the town of Silistra, but this decision greatly antagonized both countries and sowed the seeds of further enmity between them.Hall, The Balkan Wars... pp.97
The formal ending of the war was marked by the signing of the Treaty of London of 1913, which awarded all Ottoman territory to the west of the Midia-Enos line, with the exception of Albania, to the Allies.Crampton, pg.134Hall, The Balkan Wars... pp.111
Bulgaria in the Second Balkan War
The treaty failed to make clear provisions for the division of the former Ottoman territories between the victors, which brought about the dissolution of the Balkan League. Geshov foresaw this outcome, which signalled the collapse of his goal of forming a permanent alliance directed against the Ottoman Empire, and resigned from his post as prime minister. He was replaced by the hard-liner Stoyan Danev.Hall, The Balkan Wars... pp.102 The new government was not willing to compromise with Bulgarian claims in Macedonia, and neither were Serbia and Greece, whose interests were frustrated by the creation of an Albanian state. Russia, which was viewed as the patron of the Balkan League, was unable to control the situation and settle the disputes between the allies. The failure of Russian diplomacy, and the Entente Cordiale among Russia, France, and Great Britain that stood behind it, was a victory for Austria-Hungary, which sought to undermine the unity between the Balkan countries. In June, the new Bulgarian government was asked by the Bulgarian General Staff to either take aggressive action or order demobilization within 10 days. The senior Bulgarian commanders were concerned by the new alliance between Serbia and Greece and the growing restlessness in the army, which had been in the field since September 1912. Danev was preparing to leave for Russia where a new attempt to solve the problem was made by Tsar Ferdinand and General Mihail Savov, who decided to make a demonstration to Serbia, Greece and the Entente by ordering two of the Bulgarian armies to attack and consolidate their positions in Macedonia on 16 June.Stavrinos pg.539 More than a day later, Danev ordered Savov to stop the fighting and the latter obeyed, despite orders for the continuation of the attack given to him by the tsar. The Serbians and Greeks, however, were not willing to let this opportunity pass and declared war on Bulgaria. Perceiving an opportunity to acquire Southern Dobruja, Romania also invaded Bulgaria. The Romanian forces met with almost no resistance and was soon followed by the Ottoman Empire, which restored its control over Eastern Thrace.Hall, The Balkan Wars... pp.117–119
The eruption of this Second Balkan War tore a rift in the relations between Bulgaria and Russia and led to the downfall of the Danev government amidst the news of Bulgarian defeats in the field. A new liberal coalition government under Vasil Radoslavov took control and immediately began seeking a diplomatic solution to the crisis, looking primarily towards Germany and Austria-Hungary for help.Hall, The Balkan Wars... pp.118–119 Direct negotiations with Serbia and Greece proved inconclusive, but following Bulgaria's offer to cede Southern Dobruja to Romania, both sides agreed to begin peace talks in Bucharest. At the same time, the Bulgarian army managed to stabilize the Serbian and Greek fronts and even go on the offensive. Bulgarian forces threatened to encircle the Greek army completely, but with the Romanians only a few kilometers from the Bulgarian capital of Sofia and the Ottomans in good position to invade the whole of southeastern Bulgaria, the warring countries concluded an armistice in July 1913.Hall, The Balkan Wars... pp.120–122
Following the cessation of hostilities, the peace talks in Bucharest resumed. The Bulgarian delegation found itself in almost complete isolation, with only the partial support of Russia and Austria-Hungary, which forced it to accept the coercive conditions of its opponents and sign the Treaty of Bucharest of 1913.Hall, The Balkan Wars... pp.123–125 The treaty required Bulgaria to cede Southern Dobruja, most of Macedonia (including the "uncontested zone" that had previously been bestowed to it by the 1912 treaty between Bulgaria and Serbia) and the town of Kavala.
The peace treaty with the Ottomans had to be dealt with on a bilateral basis. Initially, the Bulgarian diplomacy maintained the position that the question about the possession of Adrianople and Eastern Thrace was an international matter resolved by the terms of the Treaty of London of 1913, but this line soon had to be abandoned due to the lack of support by the Great Powers and their unwillingness to pressure the Ottoman Empire. The resulting Treaty of Constantinople of 1913 restored to the Ottomans most of the lands they had re-occupied during the Second Balkan War. During the talks, the Radoslavov government for the first time sought to recover and strengthen the relations with the Ottomans by discussing an alliance directed against Serbia and Greece, but no concrete results were achieved at that point.Hall, The Balkan Wars... pp.125–126
Bulgaria in the aftermath of the Balkan Wars
The outcome of the Second Balkan War negated almost all of the territorial gains that Bulgaria secured during the First Balkan War. The failed effort to bring all Bulgarians under a single national government led to a massive influx of over 120,000 Bulgarian refugees from Eastern Thrace and the parts of Macedonia that remained under Serbian and Greek rule.Michael Robert Marrus. The unwanted: European refugees from the First World War through the Cold War. Temple University Press, 2002, pg. 46. The Radoslavov government faced the uneasy task of integrating the new population and the acquired territories that were not ceded back, as well as rebuilding the economy and the military potential of the country.