Ferdinand I of Bulgaria in the context of "Central Powers"

The Tsardom of Bulgaria (Bulgarian: Царство България, romanized: Tsarstvo Bŭlgariya), also known as the Third Bulgarian Tsardom (Bulgarian: Трето Българско Царство, romanized: Treto Bŭlgarsko Tsarstvo), usually 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 revolutions of 1917–1923 were a revolutionary wave that included political unrest and armed revolts around the world inspired by the success of the Russian Revolution and the disorder created by the aftermath of World War I. The uprisings were mainly socialist or anti-colonial in nature. Most socialist revolts failed to create lasting socialist states. The revolutions had lasting effects in shaping the future European political landscape, with, for example, the collapse of the German Empire and the dissolution of Austria-Hungary.

World War I mobilized millions of troops, reshaped political powers and drove social turmoil. From the turmoil outright revolutions broke out, massive strikes occurred, and many soldiers mutinied. In Russia, the Tsar Nicholas II abdicated during the February Revolution. The short-lived liberal Russian Provisional Government was formed, but it was overthrown by the Bolsheviks in the October Revolution, which triggered the bloody Russian Civil War. Many French soldiers mutinied in 1917 and refused to engage the enemy. In Bulgaria, many troops mutinied, and the Bulgarian Tsar stepped down. Mass strikes and mutinies occurred in Austria-Hungary, and the Habsburg monarchy collapsed. In Germany, the November Revolution led to the end of the German Empire. Italy faced various mass strikes. Turkey experienced a successful war of independence. Ireland was partitioned and the Irish Free State was created. Across the world, various other protests and revolts occurred.

After the Russo-Turkish War of 1877–1878, the 1878 Treaty of Berlin set up an autonomous state, the Principality of Bulgaria, within the Ottoman Empire. Although remaining under Ottoman sovereignty, it functioned independently, taking Alexander of Battenberg as its first prince in 1879. In 1885 Alexander took control of the still-Ottoman Eastern Rumelia, officially under a personal union. Following Prince Alexander's abdication (1886), a Bulgarian Assembly elected Ferdinand I as prince in 1887. Full independence from Ottoman control was declared in 1908.

In the 1912–1913 Balkan Wars, Bulgaria initially formed an alliance with Greece, Serbia and Montenegro against the Ottoman Empire, and together they conquered a great deal of Ottoman territory. Bulgaria, however, unhappy with the resulting division of territory, soon went to war against its former allies Serbia and Greece and lost territory it had gained in the first war. The First World War (1914–1918) saw Bulgaria fighting (1915–1918) alongside Germany, Austria-Hungary, and the Ottoman Empire. Defeat led to the Treaty of Neuilly-sur-Seine (27 November 1919), in accordance with which Bulgaria lost further territory. Social problems and political instability persisted throughout the Interwar period. In the Second World War (1939–1945), Bulgaria again allied with Germany (March 1941). Although Sofia attempted to pull out of the war as the Soviet Union advanced towards its territory (1944), the Red Army invaded (September 1944), and a communist government came to power (1944–1946) and established the People's Republic of Bulgaria (1946–1990).

The House of Saxe-Coburg and Gotha-Koháry is the Catholic cadet branch of the House of Saxe-Coburg and Gotha, founded after the marriage of Prince Ferdinand of Saxe-Coburg and Gotha and Princess Maria Antonia Koháry de Csábrág. Among its descendants were the last four kings of Portugal (Pedro V, Luís, Carlos, Manuel II) and the last three Tsars of Bulgaria (Ferdinand, Boris III, Simeon II). After the change of the “House laws” by Simeon II, the present head of the house is his sister Princess Marie Louise of Bulgaria, Princess of Koháry. and her heiress apparent is her daughter Princess Alexandra-Nadejda Kohary

Boris III (Bulgarian: Борѝс III; Boris Treti; 30 January [O.S. 18 January] 1894 – 28 August 1943) was the Tsar of the Kingdom of Bulgaria from 1918 until his death in 1943.

The eldest son of Ferdinand I, Boris assumed the throne upon the abdication of his father in the wake of Bulgaria's defeat in World War I. Under the 1919 Treaty of Neuilly, Bulgaria was forced to cede various territories, pay crippling war reparations, and greatly reduce the size of its military. That same year, Aleksandar Stamboliyski of the Bulgarian Agrarian National Union became prime minister. After Stamboliyski was overthrown in a coup in 1923, Boris recognized the new government of Aleksandar Tsankov, who harshly suppressed the Bulgarian Communist Party and led the nation through a brief border war with Greece. Tsankov was removed from power in 1926, and a series of prime ministers followed until 1934, when the corporatist Zveno (Bulgarian: Звено) movement staged a coup and outlawed all political parties. Boris opposed the Zveno government and overthrew them in 1935, eventually installing Georgi Kyoseivanov as prime minister. For the remainder of his reign, Boris would rule as a de facto absolute monarch, with his prime ministers largely submitting to his will.

Prince Kiril of Bulgaria, Prince of Preslav (Bulgarian: Кирил, принц Преславски, German: Kyrill Heinrich Franz Ludwig Anton Karl Philipp Prinz von Bulgarien; 17 November 1895 – 1 February 1945) was the second son of Ferdinand I of Bulgaria and his first wife Marie Louise of Bourbon-Parma. He was a younger brother of Boris III of Bulgaria and a prince regent of the Kingdom of Bulgaria from 1943 to 1944. He was sentenced to death by the People's Court and executed on the night of 1 February 1945.