Following the terrorist attacks of 9/11, some commentators drew a parallel between al Qaeda's strike and Japan's bombing of Pearl Harbor, and between the ensuing struggle and the Second World War. There was sporadic talk of mobilization, which was easily enough dismissed as out of step with the realities of the War on Terror. Unconventional warfighters like terrorists are by definition immune to the massive concentrations of power that are the traditional object of a mobilization. At the same time, conventional war is generally regarded as oriented toward smaller, more professional, and high-tech forces fighting in demassified conflicts. (1) However, despite a great deal of hand-wringing on the part of social critics, the really difficult question was not asked: Would a World War II-scale mobilization even have been possible after 9/11 if it had been deemed an appropriate response? With a much reduced emphasis on military competition between the great powers and far less talk of American declinism today than there was 15 years ago, the ability of major economies to bear drastically enlarged military burdens has been discussed far less of late. Even Paul Kennedy, who popularized the declinist thesis in The Rise and Fall of the Great Powers in 1987, recently backed away from it. (2) Nonetheless, two major developments have set the stage for a new debate on rises in military expenditures. The first of these developments is a changed economic and fiscal outlook, specifically a return to the massive US budget deficits of the 1980s, and less exuberant economic expectations than was the case just a few years ago. The illusion of perfection at no cost has largely faded. The other development is the demands placed on US forces since 9/11, with talk of their being stretched too thin by commitments in Iraq and elsewhere becoming fairly widespread. While the possible emergence of a large peer competitor has never disappeared from consideration, other motivations for mobilization have emerged in recent years. One is the drive to terrorist-proof the United States, the final bill for which may run into the trillions. (3) Another is a more open embrace of an imperial position likely to involve more preventative and unilateral actions, as well as prolonged occupations and nation-building projects, and the extension of America's sphere of influence to new regions like sub-Saharan Africa and Central Asia. These factors already have led to the post-9/11 expansion of the defense budget and the institution of the Department of Homeland Security, hailed as the biggest reshuffling of the executive branch since 1947. They also have brought renewed discussion of conscription and the expansion of the armed forces, with bills pending in Congress to increase the US armed forces by tens of thousands of personnel over the next five years. American requirements, capacities, and investments aside, the question of military expansion also speaks to the capacities of other nations and the relationship of the United States with them, in terms of both the dangers of great power conflict and the international distribution of security responsibilities. The European Union and Japan, for instance, have been seeking to develop militaries capable of meeting enlarged international responsibilities since well before 9/11, efforts which have been extremely sluggish. Europe set for itself the relatively modest goal of being able to deploy a 60,000-strong force 3,000 miles away for one year by 2003. That target date has come and gone, and it seems unlikely that any such force will be made operational before 2012--if then. The Defense Capabilities Initiative, intended to close the growing technological gap between Europe and the United States, has been similarly disappointing. (4) Observers looking to explain that sluggishness, like Robert Kagan in his recent book On Paradise and Power, have commonly focused on the relatively pacifistic political cultures of the EU's member states. …