The final straw event that led to World War I was the assassination of Archduke Franz Ferdinand, heir to the Austro-Hungarian throne, on June 28, 1914, by a Serbian nationalist named Gavrilo Princip.
At the time, the Austro-Hungarian Empire was a complex and diverse multiethnic state that included many Slavic peoples, including Serbians, who lived in the southern part of the empire. Franz Ferdinand was known to be sympathetic to the Slavic peoples and had proposed reforms that would give them greater autonomy within the empire. This made him a target for Serbian nationalists, who feared that his policies would weaken their chances of achieving an independent Serbian state.
The assassination of Franz Ferdinand served as the catalyst for World War I in several ways. First, it triggered a diplomatic crisis between Austria-Hungary and Serbia. Austria-Hungary accused Serbia of being behind the assassination and demanded that Serbia punish those responsible and take measures to prevent such incidents from happening in the future. Serbia agreed to some of Austria-Hungary's demands but rejected others, leading Austria-Hungary to declare war on Serbia on July 28, 1914.
The outbreak of war between Austria-Hungary and Serbia set off a chain of events that quickly led to the wider European conflict. Russia, which had strong ties to Serbia, began mobilizing its military in support of Serbia, prompting Germany, which had a mutual defense pact with Austria-Hungary, to declare war on Russia on August 1, 1914. France, which had an alliance with Russia, then declared war on Germany, and Germany responded by declaring war on France and Belgium. By early August, the major powers of Europe were at war, marking the beginning of World War I.