Short Answer 
The assassination of Archduke Franz Ferdinand in June 1914 sparked World War I by inciting Austria-Hungary’s aggression towards Serbia, which declared war and drew in allied nations like Russia. Additionally, rising nationalism in the Balkans contributed to regional instability and exacerbated the conflict among ethnic groups seeking independence, ultimately leading to a larger war.
Step 1: Assassination of Archduke Franz Ferdinand
The assassination of Archduke Franz Ferdinand of Austria in June 1914 acted as the immediate catalyst for World War I. This event outraged Austria-Hungary, prompting them to take a strong stance against Serbia, which was accused of supporting the assassin. This action set off a domino effect of alliances and mobilizations across Europe.
Step 2: Serbia’s Declaration of War
Following the assassination, Serbia’s declaration of war on Austria-Hungary escalated tensions dramatically. This declaration was met with support from Russia, an ally of Serbia, leading to a wider conflict as other countries were drawn in due to existing alliances. This situation quickly spiraled out of control, forcing nations into a war they were initially not ready for.
Step 3: Rise of Nationalism in the Balkans
Increasing nationalism in the Balkan Peninsula further contributed to the tensions leading to World War I. Various ethnic groups, seeking independence from larger empires, created a volatile environment ripe for conflict. The aspirations of these groups often clashed, contributing to the instability that enveloped the region and played into the larger European conflict.