At 30 he moved to Rome and worked as a schoolteacher. During this time he met Elio Petri, the Future Director of Investigation of a Citizen Above Suspicion (1970), who worked as assistant to Giuseppe De Santis. Guerra was able to get his first screenwriting credit after he and Petri went to the Abruzzi mountains to find out about wolf-hunting; "Though they discovered that wolf Hunters no longer existed, De Santis went ahead anyway with the film, Uomini e Lupi (Men and Wolves, 1957)".