The Tsardom of Bulgaria, also known as the Third Bulgarian Tsardom, commonly known in English as the Kingdom of Bulgaria, or simply Bulgaria, was a constitutional monarchy in Southeastern Europe, which was established on 5 October [O.S. 22 September] 1908, when the Bulgarian state was raised from a principality to a tsardom.
Prince Ferdinand, founder of the royal family, was crowned as tsar at the Declaration of Independence, mainly because of his military plans and for seeking options for unification of all lands in the Balkans region with an ethnic Bulgarian majority (lands that had been seized from Bulgaria and given to the Ottoman Empire in the Treaty of Berlin). He and his successors were reckoned as kings internationally.
The state was frequently at war throughout its existence, lending to its nickname as "the Balkan Prussia". For several years, Bulgaria mobilized an army of more than 1 million people from its population of about 5 million, and in the 1910s it engaged in three wars – the First and Second Balkan Wars, and the First World War. Following the First World War the Bulgarian army was disbanded and forbidden to exist by the Allied Powers, and all plans for national unification of the Bulgarian lands failed.
Less than two decades later Bulgaria entered World War II on the side of the Axis powers and once again found itself on the losing side until it switched sides to the Allies in September 1944. In 1946, the monarchy was abolished, its final tsar was sent into exile, and the tsardom was replaced by the People's Republic of Bulgaria.
Despite the establishment of the Principality of Bulgaria (subject to Ottoman suzerainty) in 1878, and the subsequent Bulgarian control over Eastern Rumelia after 1885, there was a substantial Bulgarian population still living under Ottoman rule, particularly in Macedonia and Adrianople. To complicate matters, Serbia and Greece too made claims over parts of Macedonia, while Serbia, as a Slavic nation, also considered Macedonians as belonging to the Serbian nation. Thus began a five-sided struggle for control of these areas which lasted until 1912. In 1903 the Ilinden–Preobrazhenie Uprising broke out in Ottoman Macedonia, making war seem likely.
In the Aftermath of the Young Turk Revolution, Ferdinand used the struggles between the Great Powers to break free from Ottoman suzerainty. On 5 October 1908, Ferdinand declared the Independence of the Tsardom of Bulgaria at the Holy Forty Martyrs Church in Veliko Tarnovo, although since the introduction of the Gregorian calendar in 1916, this has been celebrated on September 22. Even before then, however, Bulgaria had only acknowledged the overlordship of the sultan formally. Since 1878, Bulgaria had had its own constitution, flag, and anthem, and conducted a separate foreign policy.
Ferdinand took the Bulgarian title "Tsar" in honor of the rulers of the First and Second Bulgarian Empires. However, while previous Bulgarian "tsars" were reckoned as emperors, Ferdinand and his successors were called "kings" outside Bulgaria. The Tarnovo Constitution was retained, with the word "tsar" replacing the word "prince" or ''knyaz''.
In 1911 the Nationalist Prime Minister Ivan Geshov set about allying with Greece and Serbia, and the three allies agreed to put aside their rivalries to plan a joint attack on the Ottomans.
In February 1912 a secret treaty was signed between Bulgaria and Serbia, which was firstly against Austria-Hungary, but also then redirected by Bulgaria to be against the Ottoman Empire and in May 1912 a similar treaty was signed with Greece. Montenegro was also brought into the pact. The treaties provided for the partition of Macedonia and Thrace between the allies, although the lines of partition were left dangerously vague. After the Ottomans refused to implement reforms in the disputed areas, the First Balkan War broke out in October 1912.
The allies had astonishing success. The Bulgarian army inflicted several crushing defeats on the Ottoman forces and advanced threateningly against Constantinople, while the Serbs and the Greeks took control of Macedonia. The Ottomans sued for peace in December. Negotiations broke down, and fighting resumed in February 1913. The Ottomans lost Adrianople to a joint Bulgarian and Serbian task force. A second armistice followed in March, with the Ottomans losing all their European possessions west of the Midia-Enos line, not far from Istanbul. Bulgaria gained possession of most of Thrace, including Adrianople and the Aegean port of Dedeagach (today Alexandroupoli). Bulgaria also gained a slice of Macedonia, north and east of Thessaloniki, but only some small areas along her western borders.
Bulgaria sustained the heaviest casualties of any of the allies, and on this basis felt entitled to the largest share of the spoils. The Serbs, in particular, did not see things this way and refused to vacate any of the territories they had seized in northern Macedonia (that is, the territory roughly corresponding to the modern Republic of North Macedonia), stating that the Bulgarian army had failed to accomplish its pre-war goals at Adrianople (i.e., failing to capture it without Serbian help) and that the pre-war agreements on the division of Macedonia had to be revised. Some circles in Bulgaria inclined toward going to war with Serbia and Greece on this issue. In June 1913 Serbia and Greece formed a new alliance, against Bulgaria. The Serbian Prime Minister, Nikola Pasic, told Greece it could have Thrace if Greece helped Serbia keep Bulgaria out of the Serbian part of Macedonia, and the Greek Prime Minister Eleftherios Venizelos agreed. Seeing this as a violation of the pre-war agreements, and discreetly encouraged by Germany and Austria-Hungary, Tsar Ferdinand declared war on Serbia and Greece and the Bulgarian army attacked on June 29. The Serbian and the Greek forces were initially on the retreat on the western border, but they soon took the upper hand and forced Bulgaria into retreat. The fighting was very harsh, with many casualties, especially during the key Battle of Bregalnica. Soon Romania entered the war and attacked Bulgaria from the north. The Ottoman Empire also attacked from the southeast. The war was now lost for Bulgaria, which had to abandon most of her claims of Macedonia to Serbia and Greece, while the revived Ottomans retook Adrianople. Romania took possession of southern Dobruja.
In the aftermath of the Balkan Wars, Bulgarian opinion turned against Russia and the Western powers, whom Bulgarians felt had done nothing to assist them. Bulgaria, Romania, and Greece initially remained neutral, observing the course of the war before deciding where their sympathies lay.
Under the government of Vasil Radoslavov, Bulgaria aligned itself with Germany and Austria-Hungary, despite this entailing becoming an ally of the Ottomans, Bulgaria's traditional adversary. However, Bulgaria had relinquished its claims against the Ottomans, while Serbia, Greece, and Romania (allies of the UK and France) held territories perceived as Bulgarian by Bulgaria. Recuperating from the Balkan Wars, Bulgaria remained neutral during the first year of World War I. When Germany pledged to restore the boundaries of the Treaty of San Stefano, Bulgaria, boasting the largest army in the Balkans, declared war on Serbia in October 1915. Subsequently, the UK, France, Italy, and Russia declared war on Bulgaria.
Bulgaria, allied with Germany, Austria-Hungary, and the Ottomans, achieved military victories against Serbia and Romania. They captured much of Macedonia (including Skopje in October), advanced into Greek Macedonia, and seized Dobruja from the Romanians in September 1916. However, the war quickly became unpopular among the majority of Bulgarians, who endured severe economic hardship and resented fighting against their fellow Orthodox Christians while allied with the Muslim Ottomans. Aleksandar Stamboliyski, leader of the Agrarian Party, was imprisoned for his opposition to the war. The Russian Revolution of February 1917 significantly impacted Bulgaria, spreading antiwar and anti-monarchist sentiments among troops and in cities. In June, Radoslavov's government resigned, mutinies erupted in the army, Stamboliyski was released, and a republic was proclaimed.