What was the primary cause of World War I?

The primary cause of World War I was the assassination of Archduke Franz Ferdinand of Austria-Hungary. His assassination in June 1914 set off a chain reaction of events that ultimately led to the outbreak of the war. This pivotal moment heightened existing tensions among the major European powers and triggered a conflict involving multiple nations due to a complex web of alliances and rivalries.

Ferdinand's assassination by a Bosnian Serb nationalist fueled nationalist sentiments and was used as a pretext by Austria-Hungary to take a hard line against Serbia. This action prompted Austria-Hungary to issue an ultimatum to Serbia, which, when not fully accepted, led to the declaration of war. The intertwined alliances, such as those formed in the Triple Alliance and Triple Entente, meant that what could have remained a localized conflict quickly escalated into a global war.

The other choices, while significant events in the context of the war, did not directly cause the war's ignition. The invasion of Belgium occurred later, as part of Germany's war strategy, while the sinking of the Lusitania became a catalyst for U.S. involvement in the war in 1917 rather than an immediate cause. The formation of the Triple Alliance did contribute to the underlying