Leviathan Theme

I recently read a book called Leviathan, and at the same time in my english class we began discussing "theme". My teacher told us that there is one main theme in a novel, and that it can be completely summed up in one phrase, or even one word. Is this really always true?
In the book Leviathan, the underlying theme I notice is war, but how can you put all of the elements of war into one phrase? Th

Leviathan Theme