We all know the devastating cost of World War I on the lives of millions of service people around the world, but did you know that millions of horses were killed as well?
Back then, war horses were the main way that soldiers got around battlefields.
Horses from farms, racecourses and stables were often sent away to be used in combat.
That's the subject of new movie War Horse - based on the book of the same name by Michael Morpurgo.
Joe's been looking at the story of war horses.
Pictures from DreamWorks Pictures and British Pathe.