“How much is enough?” is a perennial question often put to national security and defence requirements. The largely invisible and more important extension of that question is to understand what purposes military force must serve in the first place. ‘For what’ must be the starting point for any excursion on future military capability and where, why and how force succeeds or fails. During half the Cold War, ‘for what’ was defined principally as deterring and, if war came, defeating the Soviet Union and (until the Nixon era) Red China — while retaining the capacity to fight and win a separate ‘half war’. Called the ‘2½ war’ strategy, as we learned in Vietnam, that half war exceeded our national capacity, challenging the basic assumptions for fighting two big wars. Since then, ‘for what’ meant planning for a specific number of simultaneous major or minor contingencies including peacetime deployments and was usually fiscally constrained. After September 11, enormous increases in defence spending for the Afghan and Iraqi conflicts provided a surfeit of military capacity. That said, how should the ‘for what’ question regarding future forces be answered and then put into practice given the many uncertainties and potential crises that lie ahead? First, the overarching purpose of military force in line with national interests must be to affect, influence and ultimately control the will and perception of an adversary or potential target, understanding that military force works best (and usually only succeeds) in defeating and deterring other, like military forces. Stretching the military mission to areas where it has less traction such as nation building or the war on terror must be avoided or strongly caveated. This construct was the basis for ‘shock and awe’ of the late 1990s. The threat of and more sophisticated uses of force to enhance and empower allies or to induce others to act in our interest, are part of shock and awe provided (near) perfect knowledge of the adversary is achieved and operations are performed to a standard of brilliance ensuring that however used, force works. Second, a complete spectrum for military force ranging from nuclear war and its deterrence to humanitarian intervention needs to be specified. Potential contingencies such as conflict in Korea, the Gulf or, as some argue, making China a threat-in-waiting, are part of this spectrum understanding that most crises are largely unpredictable and come as nasty surprises or indeed are over-exaggerated as dangers such as Iraqi weapons of mass destruction. Third, against this spectrum, active, reserve and regenerative forces can be matched to cover that continuum. In some cases, gaps can be filled with innovative and imaginative alternatives or deferred until actually needed. For example, rather than replace nuclear submarines and aircraft carriers on a one for one basis, some could be transferred to a cadre or inactive status as insurance for future contingencies. Clearly, sustaining critical parts of the defence industrial base, perhaps better renamed a defence intellectual property base, is important and not always cheap. Likewise, added high intensity warfare capability beyond the two corps can be placed in guard and reserve units. Last, defence spending will (and should) be cut significantly and must be done rationally to preserve capability and morale: a huge supposition. Finally and irrespective of future force levels and spending, the key to a future military is ensuring that people are the most important priority and so treated in budgets. This, in turn, mandates transforming to a brains- or knowledge-based strategy to cope with a very demanding global environment as compensation for fewer numbers in which we must think and not spend our way clear of danger. As to sizing, a smaller professional active duty force combined with the capacity for reconstitution and regeneration should new threats emerge becomes the future foundation. The ability to deploy corps-sized forces of about 100,000 from each coast to global regions of strategic importance in 30-60 days should serve defence and deterrence purposes. If the US rebalancing towards Asia continues, then forward basing in Hawaii is also a possibility. Using a generous five to one ratio for sustaining the 200,000 sized corps and routine deployments, about one million active duty personnel are needed — a 1/3 reduction from today’s 1.5 million. And using relatively small units, for partnership capacity building and supporting allies, common security interests can be strengthened. With a rational build down, an annual defence budget of about $ 400-450 billion should suffice. The crucial element is implementing a brains- and knowledge-based force. This requires a fundamental if not revolutionary shift in recruiting, educating and training our forces by applying knowledge and learning to sustain the ability for brilliance in operations while providing far greater understanding of the difficult environments in which these forces will serve. The writer is the chairperson of the Killowen Group that advises leaders of government and business and Senior Advisor at Washington DC’s Atlantic Council