This book is called, "Leviathan." It's by Scott Westerfeld, or whatever his last name is. You can also find his books The Uglies, Pretties, Specials, and the Midnighters.
This book mixes some of the real history behind World War I AND adds some of the author's own creations. One of the main characters is the main cause of the war (well, his parents and the Germans were, but as long as he's hidden, the world's safe). So he goes into hiding, and as we're following him, we're also following a girl who got into the army by pretending to be her own cousin. See, the main war is between the Darwinists and the Clankers. The Clankers' weapons and pretty much their lifestyle is metal. The Darwinists follow Charles Darwin's beliefs and constantly shy away from any automaton or machine. Anyways, so you follow these two along until they meet up by accident, and the story ends on a cliffhanger because it was printed 2009, and they're making a second book XD
But it's a really good book, and I think most of the people on this site would enjoy it.