The Constitution of the United States pronounces that one of the primary purposes of the American government is to “provide for the common defense.” The country's national security strategy is the plan for how its defense institutions intend to protect its citizens. This plan consists of guiding principles which change over time with global trends and the power of the country itself.
The principles in the national security strategy determine the specific actions the United States will take in defense matters. This is similar to the way in which a general's strategy for winning a battle will determine the individual orders he or she will issue to his or her troops. By laying out national priorities and identifying prominent threats, the actions the United States takes in defense matters work towards a common set of security goals. These priorities are set by the President of the United States, in consultation with his national security advisers and the country's Department of Defense.
America's national security policy is largely influenced by the country's military and economic power at a particular moment in time. When the country is less powerful in comparison to other countries, its strategy will emphasize avoiding conflict. President George Washington, the first American president, made a point of keeping the young United States from being drawn into the conflicts of more powerful European nations. Many years later, American presidents who presided over a much more powerful country entered two world wars to promote the nation's security.
Diplomacy and the country's choice of allies are also shaped by its national security strategy. Those countries that are more helpful in confronting what the United States identifies as present threats to its security will receive preferential treatment in foreign aid, military aid, and other diplomatic negotiations. As these perceived threats change, so do the alliances on which the nation places the most value.
The threats that its national security strategy identifies as being the most prominent will determine the nature, or attitude, assumed by the United States military. During the Cold War, when the nation identified a conventional conflict with member nations of the Soviet Bloc as a leading threat, the nation built up a large military that was trained and equipped to fight conventional battles. In the early part of the 21st century, when the nation was fighting insurgencies against governments it was supporting, the United States shifted its training and resources to focus on asymmetrical warfare.