When World War I began, Bulgaria declared strict neutrality, but the tsar and a Germanophile government under Vasil Radoslavov encouraged both sides to bid for Bulgarian intervention. In this contest, the Central Powers (Austria-Hungary and the German Empire) could offer far more at the expense of Serbia, Greece, and, later, Romania than could the Triple Entente (an alliance of Great Britain, France, and Russia), which had to take the interests of its smaller allies into account. During the summer of 1915, when the military balance swung in Germany’s favour, Bulgaria committed to the Central Powers and declared war on Serbia on October 1 (October 14). Some of the neutralist and pro-entente political figures objected, but none went as far as the Agrarian leader Stamboliyski, who threatened the tsar and issued a call for the troops to resist mobilization. For these acts he was arrested and condemned to life imprisonment.

By the autumn of 1918, approximately 900,000 Bulgarian men, nearly 40 percent of the male population, had been conscripted. The army suffered 300,000 casualties, including 100,000 killed, the most severe per capita losses of any country involved in the war. In the interior, bad weather and the absence of adult male labour cut grain production nearly in half, while those in the towns suffered from shortages of food and fuel and from runaway inflation. “Women’s riots” for food began early in 1917 and continued to the end of the war. The revolutions in Russia and the hopes inspired by American intervention in the war and by U.S. Pres. Woodrow Wilson’s Fourteen Points peace plan seemed to promise change for Bulgarians and further contributed to the breakdown of civilian order and military discipline. In June 1918 the replacement of the pro-German Radoslavov by Alexander Malinov, a leader of the parliamentary opposition, raised hopes for an end to the war, but instead frustration increased as Malinov yielded to Tsar Ferdinand’s determination to fight on.

On September 15, 1918 (New Style), the Allied forces on the Macedonian front broke through the Bulgarian lines at Dobro Pole. The army dissolved, as many of the troops deserted to return home, and others began a march on Sofia to punish the tsar and party leaders responsible for the war. Ferdinand turned to Stamboliyski, releasing the Agrarian leader from prison in return for his promise to use his influence to restore order among the troops. Stamboliyski, however, joined the uprising and, at the village of Radomir, where rebel troops were encamped, proclaimed Bulgaria a republic. The Radomir Rebellion was short-lived, as the Agrarian-led assault on Sofia was repulsed by German and Macedonian forces that remained loyal to the tsar. But this provided only a temporary respite. The Bulgarian government asked the Allies for an armistice, which was signed on September 29. Four days later Tsar Ferdinand abdicated in favour of his son Boris III and left the country.

Bulgaria was punished for its part in World War I by the Treaty of Neuilly, which assigned the southern portion of the Dobruja region to Romania, a strip of western territory including Tsaribrod (now Dimitrovgrad) and Strumica to the Kingdom of Serbs, Croats, and Slovenes (subsequently called Yugoslavia), and the Aegean territories gained in the Balkan Wars to the Allies, who turned them over to Greece at the Conference of San Remo in 1920. Bulgaria also was disarmed and subjected to a heavy burden of reparations.