What was the immediate cause of world war i in Europe?

The immediate cause of World War I in Europe was the assassination of Archduke Franz Ferdinand of Austria-Hungary on June 28, 1914, by Serbian nationalist Gavrilo Princip. This event triggered a series of political crises and alliances, leading Austria-Hungary to issue an ultimatum to Serbia. When Serbia's response failed to satisfy Austria-Hungary, it declared war on July 28, 1914, setting off a chain reaction of mobilizations and declarations of war among the major powers of Europe.