It’s from Sam Mendes, who gave us the somber Road to Perdition and the blackly funny American Beauty, so don’t expect “Flight of the Valkyries” and stuff blowing up real good… unless you expect the tropes of military movies to be deployed with knowing irony. Based on the book by U.S. Marine sniper Anthony Swofford about his experiences in the first Gulf War, this bleak flick sears a wicked swath through the bitter truth of soldiering: It is a dehumanizing venture, one characterized chiefly by numbing boredom and loneliness punctuated by moments of sheer terror — and the terror is such a welcome relief from the hell of tedium that a soldier embraces it willingly. Jake Gyllenhaal (The Day After Tomorrow), after a string of astonishingly mature portrayals of adolescents, turns in his first great adult performance as Swofford, who’s slowly being driven over the edge by his impetuous decision to join the Corps. And Mendes extends his reputation for thrillingly visual storytelling, here finding the matter-of-fact horror in the brand-new apocalyptic specter created by the Gulf War: the nightmare of a desert’s worth of oil wells afire, turning day into night and showering the world with black rain. “The earth is bleeding,” Swofford whispers, in shocked awe at his baptism by oil — it’s a reminder that Swofford’s nightmare is not his alone but belongs spiritually to the whole planet.