Although not a principle of war in itself, the role of military intelligence weaves a thick seam through each of the ten enduring principles. During the era of the musket and sword, intelligence gathering and analysis was undoubtedly more straightforward than it is today. Nevertheless, the intelligence product was, and remains, crucial to the commanders’ decision-making process. Wellington understood this implicitly. Intelligence gathering during the Peninsular War (1808–1814) was entirely what is termed human intelligence—it was intelligence gathered by people (and not to be confused with the intellectual prowess of humans). Here, there were two quite distinct networks: battlefield intelligence and a government-sponsored network of espionage and counterespionage. These are known collectively as military intelligence.

In Spying for Wellington, Huw Davies looks at both networks in considerable detail, and this is the book's strength. I am unaware of a single work on the British army's battlefield intelligence during the Iberian campaigns. It is a subject that is normally swept up, often as an afterthought, as part of a more general book on Wellington's army. There are, however, a few works that have concentrated exclusively on the business of spies and espionage. Robert Holtman's Napoleonic Propaganda (1950) provides a rather dated and brief examination. A more recent work, Elizabeth Sparrow's Secret Service: British Agents in France, 1792–1815 (1999), covers the subject in great detail but seemingly fails to address the significance of low-level espionage activity to strategic level decision making. Two other books, Jock Haswell's The First Respectable Spy: The Life and Times of Colquhoun Grant, Wellington's Head of Intelligence (1969) and Mary McGrigor's Wellington's Spies (2015), cover the subject from the perspective of a few of the intelligence (espionage) operatives in a single Napoleonic theater of operations, namely Iberia. McGrigor also inclines to romanticize the role these men played and tends to get stuck in the tactical detail at the expense of the operational and strategic significance.

Davies, even though he is principally concerned with military intelligence during the Peninsular War, looks at the subject more holistically and does not fall into the same trap. He begins by looking at the British government's strategic intelligence-gathering operation throughout Europe during the period of the French Wars (1793–1815). This was a far from seamless affair and often resulted in poor or misguided strategic decision making, as the numerous failed operations in the early years of the wars aptly demonstrate. This aspect is covered well. It is also essential in order to understand what follows. Davies goes on to examine the development of intelligence gathering and analysis during the Peninsular War, his main focus.

Davies examines military intelligence in the Peninsular War under three headings. The first is an exploration of the development of military intelligence under the Duke of Wellington. He begins with an outline of Wellington's time in India and explains how his experiences there shaped his understanding of the need for accurate and timely intelligence. Once he arrived in Iberia, Wellington was determined to establish an extensive human intelligence network that covered both battlefield intelligence and espionage. In the second section, Davies examines the organization of these intelligence networks at the strategic level, through diplomats (including two of Wellington's brothers) and government-sponsored spies, before considering the workings and machinations of the operational and tactical battlefield intelligence executed from his headquarters. The establishment of these human networks was all the more remarkable as Britain and Spain were not natural allies. Throughout the conflict, suspicion and animosity bubbled just below the surface, undermining the foundation of trust implicit to good military intelligence. The third section covers the numerous intelligence sources, the analysis processes used by and available to Wellington and the impact of all of this on his Peninsular campaigns. Davies highlights the fact that the analysis was (largely) conducted by groups of untrained staff officers, whose achievements have largely gone unrecognized. This latter section is covered well and reinforces the reader's impression of Davies's excellent knowledge of the War in the Peninsula as a whole.

Woven into all sections is an examination of counterespionage, propaganda, and code-breaking. There is a separate section, in the penultimate chapter, on topographical intelligence, another rather neglected area of interest. Davies finishes with a look forward to the Waterloo campaign in 1815, where Wellington's intelligence apparatus, having been largely dismantled following the peace in 1814, was to let him down. This failure served to reinforce the complexity and challenges of establishing working and reliable intelligence networks, on the one hand, and a working and reliable intelligence staff on the other.

This is a multifaceted subject, and Davies gives it a good airing. There are, perhaps, two areas that he could have examined in greater depth. The first is the advantage of Royal Navy sea supremacy that Wellington enjoyed and, with it, the ability to transfer and receive intelligence more rapidly and securely than were the French. Secondly, he could have included more on the implications of intelligence gleaned outside Iberia on the Iberian campaign. Lord William Bentinck's operations in Sicily and subsequently the east coast of Spain is a case in point.

Overall, this is an excellent, all-round study of an area that has hitherto been misconstrued or dealt with piecemeal. Spying for Wellington fills knowledge gaps, highlights new areas of study and is an essential addition to understanding the way that Wellington fought a highly successful series of campaigns in the Iberian Peninsula.