Projections of the rise and fall of nations form an indispensable element in the conduct of diplomacy, yet such estimates of what power is and how it is changing are very uncertain. Some observers, indeed, have suggested that we are nearly destined to get it wrong. The English statesman Henry St. John, Lord Bolingbroke, noted that those "who are in the sinking scale do not easily come off from the habitual prejudices of superior wealth, or power, or skill, or courage, nor from the confidence that these prejudices inspire. They who are in the rising scale do not immediately feel their strength, nor assume that confidence in it which successful experience gives them afterwards."
There are undoubtedly perils associated with either underestimating or overestimating one's power. The former mistake may lead to departures from an otherwise sound position or to a failure to exploit opportunities. The latter mistake is also familiar. History is littered with the examples of states that, acting on the "habitual prejudices of superior wealth, or power, or skill, or courage", undertake enterprises that prove the cause of their undoing. Though common sense suggests that these opposing traits would seldom be found together, historical experience often finds them joined at the hip. In Vietnam, for instance, both errors were committed. At the outset of American involvement, U.S. policymakers overestimated their ability to beat an ostensibly third rate power into submission and underestimated the true strength of the American position vis-à-vis the communist powers, mistakenly believing that if Vietnam turned communist the entire structure of peace would crumble.
