Budgetary necessity may have been the mother of President Barack Obama's reinvention of military strategy, but that doesn't mean the change is reckless or even imprudent. After the U.S. withdrawal from Iraq and with the winding down of the American presence in Afghanistan, it's time for new thinking.In an appearance Thursday at the Pentagon, Obama unveiled the recommendations of a Defense Department study group he said would produce a military that is "agile, flexible and ready for the full range of contingencies and threats." That would be accomplished by smaller increases in defense spending, a policy telegraphed by Obama's 10-year budget projections for fiscal 2012, which were $105 billion less than his 2011 blueprint.Although the need for greater austerity forms the background for the new directions proposed in the report, they are convincingly justified by its analysis of geopolitical trends. It proposes a reorientation of defense planning to China and the Middle East, noting that most European countries "are now producers of security rather than consumers of it." It also foresees greater attention to security threats in Africa and Latin America, though there as elsewhere the United States will develop "innovative, low-cost and small-footprint approaches to achieve our security objectives, relying on exercises, rotational presence and advisory capabilities." Finally, the report contemplates, in cautious terms, a reduction in spending on nuclear weapons, saying it's possible that "our deterrence goals can be achieved with a smaller nuclear force, which would reduce the number of nuclear weapons in our inventory as well as their role in U.S. national security strategy."The critical response to the report has focused on whether it effectively renounces the strategy that requires the United States to be sufficiently prepared to wage two ground wars simultaneously. It does not endorse such a change of policy. In fact, it declares: "Even when U.S. forces are committed to a large-scale operation in one region, they will be capable of denying the objectives of â?? or imposing unacceptable costs on â?? an opportunistic aggressor in a second region."Nevertheless, there is a discernible change in emphasis. The report grounds preparedness to fight two wars in the concept of "reversibility" â?? defined as the ability to "make a course change that could be driven by many factors, including shocks or evolutions in the strategic, operational, economic and technological spheres." The concept needs fleshing out, but if nimbleness in mobilization can be substituted for standing numbers of troops, that would be a military as well as a budgetary advantage, and both would help make the nation more secure.