From the earliest times, commanders have sought knowledge of the enemy, his strengths and weaknesses, his dispositions and intentions. But how much effect, in the ''real time'' of a battle or a campaign, can this knowledge have? In this magisterial new study, the author of A History of Warfare goes to the heart of a series of important conflicts to develop a powerful argument about intelligence in war. Keegan''s narrative sweep is enthralling, whether portraying the dilemmas of Nelson seeking Napoleon''s fleet, Stonewall Jackson in the American Civil War, Bletchley as it seeks to crack Ultra during the Battle of the Atlantic, the realities of the secret war in the Falklands or the numerous intelligence issues in the contemporary fight against terrorism.