THE LONGEST WAR; The Iran-Iraq Military Conflict by Dilip Hiro (Routledge: $16.95, paper; 313 pp.) . The eight-year-old conflict between Iran and Iraq--this century's longest declared war--also is its least chronicled, no doubt because of its futility; more than a million people were killed without either side winning any significant territorial concessions. Pakistani journalist Dilip Hiro takes us well beyond images of Khomeini dart boards and Saddam voodoo dolls to reveal calculating interest groups whom the West might have manipulated more skillfully. "The Longest War" has a colorless, dutiful tone, but unlike most news reporting, it captures the human motivations behind the war.