Is War Horse Based on a True Story?

I've been reading a book called War Horse, whose content depicts the World War I and I was wondering whether it's based on a true story. I think that War Horse has some elements from reality and some of them are fiction, but you can't determine whether it's based on a true story or not.
After I finished reading War Horse, I was surprised that it was never mentioned if the book was based on real events or if it's not actually a true story.

War Horse is a work of fiction and it is not a true story. They have made a movies about it and Steven Spielberg is the director.
Although some of the events that take place in War Horse may seem real, which makes the book look like it's based on a true story, the reality is that everything you read is just the imagination of the writer.
It will be really awesome if War Horse was actually part of reality and everything on the story was true but unfortunately, it's just another novel.

I read the answer but I was told in class today by my teacher that War Horse is indeed based on a true story that took place many years ago, so now after reading this I'm confused.
Is it a true story or not?