Based on five years of archival research, this book offers a radical reinterpretation of Britain and Spain's relationship during the growth, apogee and decline of the British Empire. It shows that from the early nineteenth century Britain turned Spain into an 'informal' colony, using its economic and military dominance to achieve its strategic and economic ends. Britain's free trade campaign, which aimed to tear down the legal barriers to its explosive trade and investment expansion, undermined Spain's attempts to achieve industrial take-off, demonstrating that the relationship between the two countries was imperial in nature, and not simply one of unequal national power. Exploring five key moments of crisis in their relations, from the First Carlist War in the 1830s to the Second World War, the author analyses Britain's use of military force in achieving its goals, and the consequences that this had for economic and political policy-making in Spain. Ultimately, the Anglo-Spanish relationship was an early example of the interaction between industrial power and colonies, formal and informal, that characterised the post-World War Two period. An insightful read for anyone researching the British Empire and its colonies, this book offers an innovative perspective by closely examining the volatile relationship between two European powers.