What event set off the first declaration of war in world war 1 was?

The first declaration of war in World War I was triggered by the assassination of Archduke Franz Ferdinand of Austria-Hungary on June 28, 1914. This event led Austria-Hungary to issue an ultimatum to Serbia, which, when only partially accepted, resulted in Austria-Hungary declaring war on Serbia on July 28, 1914. This declaration set off a chain reaction of alliances and treaties, rapidly escalating the conflict into a full-scale war.