The Second World War confirmed America's ascent to great power status. The nation emerged out of the war with great wealth, unparalleled military capabilities and a series of bases spread around the world. This was not just a traditional territorial empire exercising significant and in some places direct control over foreign possessions; the United States also commanded huge indirect global influence, primarily through its role in shaping the post-war international architecture. It had vast commercial interests and its dominance was apparent, not only in traditional metrics of power but in cultural and ideological terms too. Of course, large swathes of the globe were firmly within the Soviet orbit or otherwise under the influence of regimes subscribing to forms of communist ideology, which American institutions, investments and ideas struggled to penetrate. But this applied mainly to the ‘wasted landscapes of Europe's devastated east’ – the USSR was in disarray in the years after the war, and other communist movements were fighting bitter struggles for survival. American hegemony was the dominant fact of the post-war order, and this remained a constant of world politics up to the present day.

Preoccupied with a convergence of intersecting geopolitical, economic and ideological concerns, the country was led by men who believed America required a preponderance of power to protect and promote its vital interests. Porter usefully identifies four key factors that compelled policymakers to aggressively defend American primacy in the new geopolitical context: the nation's growing power after the Second World War encouraged it to ‘pursue security through expansion’; various crises and strategic shocks – such as Pearl Harbour or the North's invasion of South Korea in 1950 – inspired a sense of growing vulnerability requiring preventive action beyond its shores; tempted by its growing power, America's universalizing liberal traditions encouraged it to shape the world in its own image, confident that greater security would come from a world community of like-minded democratic republics; and finally, a ‘self-propelling dynamic of empire’ meant its growing global purview created new frontiers and with it new insecurities, commitments and anxieties. The practical measures this all entailed required America to overturn decades of foreign policy shibboleths warning about large military establishments, entangling alliances and going abroad seeking monsters to slay.