Defense Primer: Geography, Strategy, and U.S. Force Design [Updated September 13, 2019] [open pdf - 437KB]
From the Document: "Most of the world's people, resources, and economic activity are located not in the Western Hemisphere, but in the other hemisphere, particularly Eurasia. In response to this basic feature of world geography, U.S. policymakers for the last several decades have chosen to pursue, as a key element of U.S. national strategy, a goal of preventing the emergence of regional hegemons in Eurasia. This objective reflects a U.S. perspective on geopolitics and grand strategy developed by U.S. strategists and policymakers during and in the years immediately after World War II that incorporates two key judgments:  that given the amount of people, resources, and economic activity in Eurasia, a regional hegemon in Eurasia would represent a concentration of power large enough to be able to threaten vital U.S. interests; and  that Eurasia is not dependably self-regulating in terms of preventing the emergence of regional hegemons, meaning that the countries of Eurasia cannot be counted on to be able to prevent, though their own actions, the emergence of regional hegemons, and may need assistance from one or more countries outside Eurasia to be able to do this dependably."
CRS In Focus, IF10485
Congressional Research Service: https://crsreports.congress.gov/