Throughout the post-Vietnam era, both the main strands of exceptionalist belief have been influential on the discourse and content of U.S. foreign policy. Indeed, the tension between these two strands has defined many of the debates over the post-Vietnam direction of foreign affairs, particularly with regard to foreign interventions and the use of force. The United States has remained highly engaged in international affairs, but with added pressure that its conduct be exemplary and consistent with its traditional values and principles. Policymakers and public alike have sought to avoid "another Vietnam" by ensuring that foreign policy actions are conducted in keeping with the perceived lessons of that conflict. The defeat of U.S. objectives by a technologically inferior enemy in Vietnam indicated that there were limits to American power. The nature and extent of these limits, however, continues to be a major source of debate, and has come to dominate discussions over foreign policy in each post-Vietnam administration. The term "Vietnam syndrome" became widely used to describe the collective lessons and legacies of the war, particularly in the political-military realm. Although there is no nationwide consensus on the lessons of the Vietnam War, a pattern has developed in policymaking that has remained relatively consistent across post-Vietnam administrations, especially in the threat and use of force. The Vietnam syndrome, in its political-military sense, amounts to a set of criteria that should be met if the United States is to commit troops to combat. These criteria must be satisfied if public support for military intervention is to be sustained. Presidents feel the need to maintain public support for their foreign policy largely because this grants it the moral legitimacy that became so lacking in Vietnam. To avoid another Vietnam, policymakers have therefore followed the central criteria of the Vietnam syndrome: that the United States should not employ force in an international conflict unless just cause can be demonstrated, the objectives are compelling and attainable, and sufficient force is employed to assure a swift victory with a minimum of casualties.
These conditions for the use of force form the content of the Vietnam syndrome and have become increasingly institutionalized with each successive administration. They have been codified in the Weinberger Doctrine, proposed by Reagan's Secretary of Defense Caspar Weinberger in November 1984, and the Powell Doctrine espoused by Colin Powell, the chair of the Joint Chiefs of Staff in the administration of George H. W. Bush and secretary of state under George W. Bush. Even though, as Powell himself has argued, administration officials do not formally go down a list checking off the specific conditions of the Vietnam syndrome, it is clear from public and archival accounts of the decision-making process that deliberate steps are taken to ensure these conditions are met before use of force is authorized. The planning and conduct of all major uses of force since the Vietnam War have been influenced directly by the Vietnam syndrome. Even the Gulf War, which George H. W. Bush claimed "kicked the Vietnam syndrome once and for all," demonstrates how strongly the syndrome's influence persists. The planning and conduct of the Gulf War, with its emphasis on an air war and in particular the decision not to pursue the war to Baghdad once the objective of liberating Kuwait had been achieved, was carefully designed to comply with all of the Vietnam syndrome's central tenets: use force only as a last resort in the pursuit of what can be demonstrated as a just cause with compelling objectives that can be achieved swiftly using maximum force with minimal casualties. Far from "kicking the Vietnam syndrome," the Gulf War helped to institutionalize it.
The Vietnam syndrome actually has the effect of reinforcing and perpetuating crucial elements of the belief in American exceptionalism. It is designed specifically to ensure that the United States does not commit itself to another conflict like the Vietnam War. A central purpose of the syndrome is to avoid situations in which the United States could suffer another military defeat. By following policy based on the Vietnam syndrome, U.S. policymakers can be reasonably assured of achieving victory, and thus reinforcing the exceptionalist belief that the United States is a superior nation which always succeeds in its objectives. The Vietnam syndrome also prevents the United States from undertaking military commitments involving long-term occupations of hostile foreign territory. This requirement perpetuates the exceptionalist notion that the United States does not seek the conquest and subjugation of foreign nations. Finally, and most significantly, if it follows the Vietnam syndrome, the United States will use force only in situations where Americans can perceive a just cause. Whenever a just cause is conceived by American policymakers, no matter whether its roots are economic, strategic, political, or otherwise, it will be couched in terms of American exceptionalism. By following what is perceived as a just cause, any administration will perpetuate the belief that the United States pursues only policy that is consistent with its exceptional values and principles. In this sense, the belief in exceptionalism is self-perpetuating and the Vietnam syndrome does nothing to change that situation; in fact, it reinforces it.
The Vietnam syndrome acts as a constraint on American action in world affairs. It places limits on the strength, resolve, and capabilities of a nation which Americans regard as all-powerful and superior to other nations. In this sense the power of the Vietnam syndrome in the making of American foreign policy suggests that the United States is no longer an exceptional nation but is just as limited in its actions as any other country. Yet, paradoxically, the Vietnam syndrome actually acts as a guarantor of the continued acceptance of the belief in American exceptionalism. If the Vietnam syndrome is followed, then the United States can continue to be at least perceived as an exceptional nation because it will always win its wars, it will remain committed to its tradition of not conquering foreign land for territorial expansion, and it will resort to force only in the pursuit of just causes. The nature of the Vietnam syndrome is a clear example, then, of how the belief in American exceptionalism continues to frame the discourse of U.S. foreign policy.