Austria-Hungary's declaration of war on Serbia after the Archduke Franz Ferdinand was assassinated precipitated World War I. Because of a tangle of alliances and treaties, other European powers declared war in a chain reaction, plunging the entire continent into war within a week.
According to Firstworldwar.com, Austria-Hungary pulled in the armies from its European empire. Poles, Czechs, Slovaks, Italians, Bosnians and Croatians all fought on the Axis side of the war. Germany eagerly entered the war as an ally, but its much greater expertise in waging war soon made it the dominant partner in the new alliance. By 1916, Austria-Hungary was following Germany's lead, and low morale coupled with an ethnically diverse and increasingly Balkanized army began to weaken the once-formidable empire.
As the war dragged on, Austria-Hungary's empire began to dissolve. By the end of the war, the Czechs and Slavs, disillusioned by their dysfunctional overlords, had declared independence. The Allies, seizing the opportunity, announced their support of these breakaway countries, encouraging more dissolution in the Austria-Hungarian empire. On Oct. 31, Austria dissolved the Austria-Hungarian union, and by the end of the war in November, the Hungarian part of the empire had lost nearly three-fourths of the territory held prior to the war.