After the turn of the century, the Bush administration focused on strengthening the “hardware” of nuclear forces in order to support its “preemptive” counter-proliferation strategy. The succeeding Obama administration has attempted to maintain the U.S. nuclear superiority with an eye to the future development of its nuclear forces. The impetus for Obama’s adjustments includes the change of nuclear security environment, the shift of focus of U.S. nuclear strategy, the impact of the revolution in military affairs, and the important role of “Instant Global Strike System ”in U.S. national security strategy. In the future the Obama administration will likely press ahead with strengthening nuclear infrastructure and human resources, but the possibility of  the U.S. taking measures to upgrade its nuclear arsenal cannot be ruled out.