…Austria-Hungary invaded Serbia and triggered the series of alliances and defense pacts that ignited the First World War.
Despite playing a role in setting off the war, both nations would become overshadowed by the larger players that immediately became involved, namely Germany, France, the U.K., and Russia.
After putting up stiff resistance for the first year, the Kingdom of Serbia was conquered during the course of 1915 and occupied by Austro-Hungarian forces until the war’s end in 1918. Serbia lost more than 1.1 million people, including 25 percent of all troops, 16-27 percent of its overall population and 60 percent of its males. Proportionally, it suffered more losses than any other country involved (in this regard, the Ottoman Empire ranks second, losing 13-15 percent of its population, followed by Romania, an Entente member, at 7-9 percent).
For its part, the weak and declining Austro-Hungarian Empire lost over 2 million people, of whom 120,000 were civilians, amounting to about 4 percent of its total population. Having exhausted itself in its pyrrhic victory against Serbia, the country barely kept it together throughout the conflict, remaining a peripheral power dependent on German support.
Indeed, Austria-Hungary would ultimately collapse into several new countries, some of which would join Serbia to form a new multiethnic state called Yugoslavia.