As another presidential election looms, the America's role in global affairs and security has emerged as one of the campaign's great battle lines. The struggle not just to define but also to preserve American power is no modern phenomenon: questions of intervention and projection have dominated the nation's politics from the days of the Founding Fathers. Then, as now, the old centres of power were shifting. Nor is economic stress an unfamiliar factor for policymakers.

But in 2012 these problems are compounded by the on-going financial crisis in Europe, which, together with the overstretch and fatigue from two wars, has sapped the strength of America's chief allies. While it may urge its NATO partners to shoulder more of the security burden, the US finds them less willing and occasionally unable to share the strain. This Adelphi examines the myriad challenges America must confront if it is to uphold and spread its values.