Many historians believe that Austria-Hungary's annexation of Bosnia and Herzegovina in 1908 directly led to the outbreak of World War I. Bosnia and Herzegovina were two provinces of the Ottoman Empire that had been occupied by Austria-Hungary in 1878. The annexation angered Serbia, which had long claimed the two provinces as its own. In response to the annexation, Serbia began to mobilize its military. Austria-Hungary, fearing a Serbian attack, mobilized its own military in response. This set off a chain of events that led to the outbreak of war in August 1914.
Other historians believe that Austria-Hungary was not solely responsible for World War I. They argue that the war was caused by a complex combination of factors, including the arms race between the European powers, the growth of nationalism, and the failure of the European diplomatic system. They also point out that Serbia had also played a role in the escalation of the crisis by mobilizing its military.
Ultimately, the question of who was to blame for World War I is a complex one that has no easy answer. It is likely that many factors contributed to the outbreak of war, and it is not possible to say definitively that any one country was solely responsible.