<human>:  What was the cause of the First World War?
<bot>: The main cause of the First World War was the assassination of Archduke Franz Ferdinand of Austria. Franz Ferdinand was the heir to the throne of Austria-Hungary. The assassination sparked a chain of events that eventually led to the war. The assassination occurred on June 28, 1914. The Austro-Hungarian Empire declared war on Serbia on July 28, 1914. On August 1, 1914, Germany declared war on Russia, and on August 3, 1914, Great Britain declared war on Germany.