The United States has played a paradoxical role in world order: . . . expanding across the continent in the name of Manifest Destiny while abjuring any imperial designs; [exerting] a decisive influence on momentous events while disclaiming any motivation of national interest; and [becoming] a superpower while disavowing any intention to conduct power politics. How do we explain this ambivalence, and what are its implications? Can we have it both ways? Does it depend?

The paradox of the United States role in the world order must be assessed from multiple standpoints. First and foremost, one must recognize the passage from old world to new world within European history. The colonist who left Europe to found America were very eclectic, largely religious, and had suffered persecution at home. These American settlers had intimate contact with other authoritarian monarchies, states dominated by religious rule and imperial republics amongst the other European powers such as; Britain, France, Germany and Spain. This intimacy with European imperialism created a dual desire for civil liberty and republican secularism.
The American Union was consolidated around conflicting interest mainly economic expansion away from Europe and greater civil liberty, versus religious freedom and republican government. Economic expansion civil liberties and religious freedom converge in manifest destiny, proving both as possible. I believe this is the beginning of America's just war doctrine. Exterminating the native population was viewed as a constituent project of expanding the frontier and protecting national interest.
As far as being a superpower yet rejecting imperialism, the constitutional republic resolves what appears to be a contradiction. Not purely an authoritarian monarchy yet administratively capable of engaging in militarism; America uses the ideal of democratic rule and popular sovereignty to hide its imperial ambitions behind a doctrine of just war and humanitarian intervention. Its easy to view this ethical dualism from the perspective of our modern war on terror; A christian democratic republic versus the Islamic terrorist threat.

The ambivalence regarding westward expansion and Manifest Destiny can be explained by sociological relationships between Native Americans and white settlers. Political officials of the era dismissed accusations of imperialism by denying personhood to Natives. In their minds, European imperialists oppressed fellow Europeans, or they physically invaded faraway nations. American settlers expanded West across the land on which they already lived. Perception explains this paradox.
However, the ambivalence regarding international politics cannot be explained by societal perception. If the United States was not driven by national interest, there would be no involvement in or influence over major events. Defense of other nations is driven purely by trade concerns and self-preservation.
In becoming a superpower, power politics are unavoidable. The United States has a long history of extending its political reach in order to increase its sphere of influence. Therefore, any claims of avoiding power politics are false. The United States cannot have it both ways. A nation cannot forcefully expand without imperialism. A nation would not influence international events were it not for national interests. A nation cannot be a superpower without conducting power politics.