The immediate cause of World War I was the assassination of Archduke Franz Ferdinand of Austria-Hungary and his wife, Sophie, on June 28, 1914, in Sarajevo. The assassination was carried out by Gavrilo Princip, a member of a nationalist group called the Black Hand, which sought to promote Serbian nationalism and resist Austro-Hungarian rule.
This event set off a chain reaction of diplomatic crises and alliances among the major powers of Europe. Austria-Hungary issued an ultimatum to Serbia, where the assassin and nationalist sentiments were rooted. When Serbia's response was deemed unsatisfactory, Austria-Hungary declared war on Serbia on July 28, 1914. This conflict quickly escalated as various countries honored their alliances and commitments, leading to a full-scale war involving many nations.