The book advances a novel theory of grand strategy; particularly it explains the key changes in the strategy over time. It applies the theory to explain the major changes in US grand strategy from Truman to Trump. The explanation highlights the effects of the international system on the selection of the grand strategy. The theory advances a new classification of four major strategies, based on a variation in the combination of objectives and means. These strategies include offensive and defensive realism and offensive and defensive ideology-promotion. The selection of which grand strategy will dominate in a certain period is brokered by the international system, more specifically the combined effect of the global distribution of capabilities and the balance of threat confronting the country. This theoretical model is applied in the book’s chapters to explain the key changes in American grand strategy from 1945 and until early 2020.