Leviathan (Book One, Leviathan Trilogy)

The year is 1914, the Great War, otherwise known as World War I, has begun. However, this isn't the normal war story you are used to hearing. The war is  fought by two factions: The Darwinists (The Allies) and the Clankers (The Central Powers). The Darwinists use fabricated animals to do their fighting,  while the Clankers use machines to power through opposition. The Clankers believe that the Darwinist creations are a violation against God, while the Darwinists hate the pollution the Clanker machines make. Aleksander is the son of the Archduke Franz Ferdinand, heir to the Austrio-Hungarian Empire. When his father and mother are murdered in cold blood, he must flee, with the faithful crew of a Clanker Stormwalker, to Switzerland in hopes of finding refuge. Deryn Sharp is a girl, a girl that has always dreamed of joining the British Air Service. There is, however, one not so small setback: women aren't allowed into the military. In an attempt to enter, she disguises herself as a boy and enlists. When Alek and Deryn's (now Dylan) paths collide, the fate of two empires, and the world, hang in the balance.

Leviathan is the first book in the Leviathan trilogy. The second is Behemoth and the third is Goliath. I loved Westerfield's version of alternate history and find it intriguing and sensational.