Prior to diplomacy is policy, which guides the diplomats in their actions; prior to policy are the ideas that inhabit the heads of the policymakers, shaping their perceptions of the world and informing their responses to those perceptions. Monarchs and dictators may manage to determine policy on the basis of narrow notions of personal self-interest, although most even of the autocratic sort persuade or delude themselves of a coincidence between self-interest and national interest. Democracies are hardly spared selfishness in their leaders, but democratic politics demand that policies be defended, even when they do not originate, in terms of national interest – of a conception of an overriding common good transcending the specific interests of parties, factions, and other entities smaller than the nation as a whole.

In American politics and diplomacy, the search for the national interest has been constant, from the founding of the Republic to the present. The first enunciation of national interest coincided with the proclamation of the existence of the United States of America, and indeed the coincidence was as much conceptual as chronological, for until the Treaty of Paris of 1783, national existence – that is, independence – was the essential national interest. National existence remained an issue through the War of 1812, as the British invasion of Washington demonstrated; it was contested in another, more deadly form in the sectional crisis that culminated in the Civil War.