Universal’s “The War” got a major escalation this week with Kevin Costner agreeing to take a supporting role in the film, playing a Vietnam vet returning home from the war.The film, written by Kathy McWorter, will be directed by Jon Avnet. It marks his first outing behind the cameras since “Fried Green Tomatoes.” While Universal exex would not confirm the deal, sources said that Costner — currently on location filming the Lawrence Kasdan-directed feature on Wyatt Earp — only agreed to do the film because it was a supporting role. Island Pictures bought McWorter’s script for $ 500,000. The pic, to be distributed by U, is set in Mississippi in the early 1970s. It centers on two groups of kids who build a treehouse/fort, with the two groups becoming enmeshed in a battle over the fort. Costner will play the father of one of the film’s stars, Elijah Wood. The older actor portrays a Vietnam vet who ultimately teaches the children about the futility of war. U plans to release the film in the fall or winter of 1994. Sources said production on the film would probably start in August. McWorter was recently commissioned to write the sequel to “Fried Green Tomatoes,” which Avnet will direct after “The War.” Costner’s TIG Prods. currently is doing business out of the Warner Bros. lot, but Warner Bros. exex had no comment on the deal.