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How did the assassination of Archduke Franz Ferdinand help trigger World War I?

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Stone ruins of Habsburg Castle, also known as Habichtsburg, stand on a hill above green vineyards in Aargau canton, Switzerland, with a flag flying atop the tower. — House of Habsburg Ruins of the castle of Habsburg, or Habichtsburg (“Hawk's Castle”), built 1020, the original seat of the house of Habsburg, Aargau canton, Switzerland.

The assassination of Archduke Franz Ferdinand on June 28, 1914, served as the immediate catalyst for World War I by providing the Habsburg empire with a long-sought pretext to eliminate the perceived threat posed by Serbia.12

The archduke, heir to the Austro-Hungarian throne, was killed in Sarajevo by Gavrilo Princip, a Serbian nationalist associated with the Black Hand, a group seeking to unite South Slav peoples and destroy Austro-Hungarian rule in the Balkans.13

Following the attack, the Austro-Hungarian government, backed by Germany, issued a deliberately provocative ultimatum to Serbia. When Serbia’s response was deemed insufficient, Austria-Hungary declared war on July 28, 1914. This action triggered the complex European alliance system, drawing in the Triple Entente powers—Russia, France, and Britain—and rapidly escalating a regional Balkan conflict into a global war.2134