War is a genre literal. He looks and acts like a solider from World War II. Though he isn't shown in the beginning he appears that Kevin called upon him for inspiration. But since he didn't show with the other genres, he was most likely used for as a type of soldier. Since Kevin, found even with the genres here, that they were boring. Bigby, Snow, Gary, and Mister Revise; along with the Page Sisters, had come to stop him from doing further damage to reality. Kevin sent the Genres, including War to guard his house and fight off the others.
Bigby, who had been put through much humiliation and transformed into a girl to further it, lost his patience. While Robin, Priscilla, and Hillary fought off the others, he snuck around trees and attacked all the Genres. Most likely War, didn't take Bigby's appearance seriously since he had been changed into a girl. That would be a mistake as Bigby would show no mercy and kill War and the rest of the Genres. For the moment, after defeating Kevin, War and the rest of the Genres had apparently recovered. Or it meant they were resurrected by Kevin or simply replaced. They were all sent into a new world where Kevin could do what he want and leave the mundy world alone.