'No war can be conducted successfully without early and good intelligence,' wrote Marlborough, and 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, which will fascinate readers of both military and more general history, 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. From the Napoleonic Wars to the sophisticated electronic warfare of the 21st century, John Keegan finds linking themes which lead to a compelling conclusion. His 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, or the polymorphous intelligence issues of the contemporary flight against terrorism.