Which factor contributed most to the outbreak of World War I?

The assassination of Archduke Franz Ferdinand is widely recognized as the immediate catalyst that triggered the outbreak of World War I. The assassination, which occurred on June 28, 1914, acted as the spark that ignited existing tensions in Europe, culminating in a complex web of alliances and militaristic posturing. Following this event, Austria-Hungary issued an ultimatum to Serbia, which, despite accepting most of its demands, did not satisfy Austria-Hungary, leading to declarations of war.

This incident highlighted the significant nationalistic sentiments and rivalries among the great powers at the time. It set off a chain reaction where alliances were activated—Russia mobilized to support Serbia, Germany declared war in defense of Austria-Hungary, and soon, other nations were dragged into the conflict. While other factors like economic instability, colonial rivalries, and the decline of monarchies played roles in creating a volatile environment, the assassination directly set off the series of actions and treaties that resulted in a global war.