War Horse is based on the novel by Michael Morpurgo, a solid young adult novel with classic status in the UK. It was turned into a brilliant bit of theater (really, you should see it if at all possible). But Spielberg's film fails on every level.
This movie is not just another sentimental story about a boy and his horse. It conveys as clearly as any movie I've seen the utter horrors of war and the moments of grace that can occur between enemies.
Many years ago I wrote a screenplay called The War Horses. My story, telling how the Boer War of Africa was won on a ranch in the American West, was based on a document discovered in the University of Denver's library.