This book is a study of the causes and the consequences of modern imperialism, with a focus on British and US imperialism in the nineteenth and twentieth centuries, respectively. It analyzes the dynamics of both formal and informal empires. It shows that, time and again, the pursuit of national economic prosperity led hegemonic powers to expand into peripheral areas of the world. Limiting the sovereignty of states on the periphery was the main method of ensuring that peripheral economies stayed open and accessible to Anglo-American economic interests. Loss of sovereignty, however, greatly hurt the life chances of people living in Asia, the Middle East, Africa, and Latin America. Sovereign states may not be sufficient to facilitate economic progress, but sovereignty is a precondition for the emergence of effective states. Effective states, in turn, are necessary to propel countries toward prosperous industrial societies. Some exceptions notwithstanding, imperialism—both formal and informal—has thus repeatedly undermined the prospects of steady economic progress in the global periphery.