In his new book Intelligence in War, John Keegan asks the question of just how useful intelligence information is in wartime. His answers are at times surprising, but well supported by fact.

Keegan examines the role played by intelligence in several campaigns, including Nelson’s pursuit of the Napoleon in the Mediterranean, Jackson’s Valley Campaign, and the pursuit of Von Spee in World War I. He also examines a number of World War II campaigns, including the Battle for the Atlantic, Crete, and Midway. There is also a chapter on military intelligence since World War II that examines intelligence in Falkland and Gulf Wars, and the recent War against al Qaeda.

Prior to the invention of wireless communications, it was extremely difficult, and often impossible, for commanders to collect military intelligence in anything close to real time. Ships of Nelson’s time were limited to searching for an enemy within line of sight. Keegan shows how even under these conditions, Nelson made skillful use of such fragmentary information as he could collect to bring the French fleet to book in Aboukir Bay. Stonewall Jackson likewise could collect little information about the enemy in real time, but he could and did make excellent use of cartographic intelligence. Keegan examines how Jackson used superior knowledge of the Shenandoah Valley to gain an advantage over the Union forces. The Valley, like much of Virginia, was unmapped at the start of the war. Jackson ordered detailed maps prepared, and these, combined with sympathetic local guides, proved extremely useful in the Valley campaign.

With the 20th Century came wireless communications and signal intelligence. Signal intelligence could provide a commander with real time information about the enemy’s strength and plans. Signal intelligence was potentially much more useful than information sent by spies. Spies have difficulty getting access to the enemy’s secrets. They are also vulnerable to capture when they try to transmit their information, and their reports frequently cannot be sent in a timely manner. They are in danger of betrayal as well. Signals can be intercepted in real time and without the enemy’s knowledge. If quickly decoded, they can provide a huge intelligence advantage.

Keegan has several chapters detailing the importance of signal intelligence in World Wars I and II. He gives a detailed account of the breaking of German and Japanese codes by the Allies, and the role this played in their eventual victory. In most cases, intelligence was only one of a number of factors that decided the outcome of a campaign. There is also an excellent chapter on the role of human intelligence in ferreting out information on the Nazi V weapons.

The chapter called Military Intelligence Since 1945 is perhaps the most interesting in the book, though Keegan admits that an examination of this subject is limited by the fact that most intelligence records of that period are still classified. Keegan still manages to give a good treatment of intelligence in the Falklands and in the Gulf. He also discusses the intelligence war against al Qaeda, and expresses some pessimism about the prospects of Western security agencies rooting out this scourge anytime soon.

In the end, Keegan reaches a sobering conclusion. Intelligence is useful in war, but is not by itself decisive. Even a large intelligence advantage cannot guarantee victory. Wars are decided by force, and to win the force must be sufficient, and skillfully and courageously applied. Nelson and Jackson often had fragmentary information about the enemy, but they were great commanders leading forces with remarkable fighting qualities. The British knew well in advance from signal intercepts of the coming German invasion of Crete. It did them no good. The island fell because the Germans outfought them. The American victory at Midway was a very near thing, even though America had broken Japan’s naval codes. The batt