A political community that achieved independent existence through armed struggle, the United States of America is founded on principles that justify and regulate the use of force. Its policies for developing and employing its capacities to use force have been defended and attacked at home in part on the basis of those principles. Geopolitical facts and technological innovations, as well as strategic and tactical imperatives common to political communities of widely differing principles, may be more important in shaping precisely how force is used and what capacities to use it are developed. But the distinctive, fundamental political principles of a political community are especially important in shaping why force is used and why the capability to use it is acquired. Why force is used in turn critically influences when, if not always directly how, it is used. Once force is used war may have its own logic, but in peacetime the ability to use force has an effect on events that depends decisively on what ends or principles are understood to justify and regulate its use.