Important as a clear understanding of the domestic and international context is for the strategist, it is still only the background for strategic thought. Strategy, after all, is not merely an intellectual but a performing enterprise; it exists for action and proves its value through results. Its first requirement is therefore a sense of direction through the informational morass presented by the environment it seeks to influence, a way of deciding what goals to pursue. Strategy must begin, in other words, with purpose; and purpose in foreign affairs strategy rests on the concept of the national interest.

The national interest's role in strategic logic is to justify the statesman's actions, to provide a standard of judgment against which goals can be measured. It is “an effort to describe the underlying rationale for the behavior of states and statesmen in a threatening international environment.” The strategist's sense of the national interest answers the question: Why are we doing this? Although it alone cannot offer a complete answer to that question, the national interest provides a first cut at the desirability of any given policy, a necessary if not sufficient condition for pursuing a goal. If a policy cannot be justified on the basis of the nation's interests, then it should be abandoned. As former Congressman Lee Hamilton put it, “The basic test for judging any foreign policy decision is easy to state but hard to apply: Does it serve the American national interest?”