The major units in international politics chiefly interact with the states in their geographical neighbourhood, even though certain units have a wider action space than others. The main thesis of this book is that nation-states' mutual non-mobility constitutes a fundamental property of international politics, on a par with its much debated anarchy (the absence of an international government). This book presents the non-mobility argument. It surveys the international relations literature, critically examines the systematic theory of Kenneth Waltz, the reductionist approaches of comparative foreign policy and small state theory and it introduces and illustrates the notion of environment polarity instead of the more usual systematic polarity. Three different theories rooted in the salient environment are formulated and tested: one about the tension between strong and weak; the second about the balance of power between the strong and the bandwagoning of the weak; and thirdly the twin distance model, which explains the five Nordic countries' respective Baltic engagements on the basis of their geographical distances from the Soviet Union/Russia and from the Baltic Sea region. From these theories the book concludes that the salient environment should be ascribed primacy in relation to internal explanatory factors, and two illustrations of internal and external interplay are offered. Finally the book investigates theoretically and empirically the precise nature of this state-centrism in relation to international government organizations and the EU.