The environment within which people exist has an impact on their interactions. For this reason, people seek to alter their environment. These statements form a systemic argument - a form of argument common in economics, health, and sociology, but exceptionally rare in international relations. This book is the first to describe and test a fully systemic theory of international politics - one in which the actors and their environment influence each other. Using advanced time-series techniques on an original dataset triangulated with three in-depth historical case studies spanning two centuries, the book traces statesmen's efforts to influence the broad contours of the international system within which they interact. The results conclusively demonstrate the existence of systemic effects; moreover, they show that those effects are inherently complex - that is, they are not reducible to the impact of any one theoretical school or paradigm.