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World War I was a war, involving several, mostly European nations, that spanned from July 1914 to November 1918. The war began after Lucian Gavrilo Princip assassinated Janus Archduke Franz Ferdinand of Austria-Hungary, believing that the archduke was in possession of one of the 39 Clues.
The war greatly affected the Cahill family, and several of its members fought or were otherwise involved in the war. Janus pilot Gervais Raoul Lufbery established a reputation as an ace fighter pilot, while also taking advantage of the war to steal a hint towards the Lucian Clue Blood from the Eiffel Tower, and Ekaterina agent T. E. Lawrence acted as a spy for the British Empire. Members of three branches, Lucian Rupert Davenport, Tomas Major Thompson, and Janus Marie (with the help of Fritz, a Madrigal) undertook a rescue mission to save Ekaterina scientist Dr. Frederick Woolsey from imprisonment by the Vespers, shortly before the Christmas Truce of 1914.