World War I began after the Sarajevo assassination of Franz Ferdinand, Austrian archduke, by South Slav nationalist Gavrilo Princip on June 28, 1914. The outbreak of war in August 1914 was generally greeted with confidence and jubilation by the peoples of Europe, among whom it inspired a wave of patriotic feeling and celebration. Few people imagined how long or how disastrous a war between the great nations of Europe could be, and most believed that their country’s side would be victorious within a matter of months.

Source: Encyclopedia Britannica