The movie tells the story of Joey, a horse raised in a bucolic English countryside who is torn away from his home -- and stable lad Albert -- and sent to France to the battlefields of World War I.
To a soundtrack heavy on violins, the moviegoer is swept into the epic struggle Albert has in finding his equine partner amid the blood, mud and misery of the Great War.
"World War I was the last hurrah for the horse (in) warfare," three-times Oscar winner Spielberg -- who also has his 3D "Adventures of Tintin: Secret of the Unicorn" out for the holidays -- told industry daily Variety.
"It was a time when the technological revolution, mainly in the implementation of new technologies to kill more efficiently and more cruelly, were supplanting the usefulness of the horse, which had brought terror into the hearts of standing armies for centuries," Spielberg said.
"And after World War I, that was over and the horse went back to a more bucolic and sane way of life. So it's really more of a story about courage and connections and less of a story about combat."
"War Horse," which is on the shortlist for the best dramatic film Golden Globe, is based on a 1982 children's book of the same name by British writer Michael Morpurgo, and the play adapted for the stage by Nick Stafford.
Almost two years ago, Kennedy was on vacation in London and went see the stage version of the story with her daughters.
"When I got home I talked to Steven (Spielberg) about it and told him what the play was about and he said 'Wow, that sounds like a story, it would make a wonderful movie'," she told AFP.
The most difficult thing, said the producer -- who worked with Spielberg on classic movies including ""E.T," "Indiana Jones" and "Schindler's List," was the use of "so many animals," she said.
"Whenever you are using animals in a movie you have to take extraordinary care, I mean, you do that to the people as well, but when you have innocent animals, it requires everybody involved being specially careful."
Joey, the real hero of the movie, was played by around a dozen horses from all from around the world, notably Spain. Stable boy Albert is played by 21-year-old British actor Jeremy Irvine, who had previously only worked in TV.
"Steven felt that he wanted to make a discovery, he wanted to bring a young actor to the role who hadn't necessarily done a lot of things in the movies," said Kennedy.
Most critics so far have been broadly positive, although some have questioned Spielberg's approach, like the Guardian's newspaper critic Andrew Pulver, who said the director "can't seem to snap out of a now-habitual mode of vitality-erasing, dewy-eyed affectation."
Todd McCarthy of the Hollywood Reporter said the film "possesses a simplicity that is both its greatest strength and an ultimate liability.
"Whatever its missteps, this is a film that kids, middle-aged adults and grandparents can all see -- together or separately -- and get something out of in their own ways," he wrote.
"There are precious few films that fit this description today, and hats off to Spielberg for making one."