These varied meanings of the frontier - as the edge of empires and creator of a nation, a stage for heroes and backdrop for adventure, a technological testing ground and spur to industry, a story of conquest and of rich cultural exchange - have all emerged from more than a century of writing on the subject.

The most influential early view of the frontier's importance came from the historian Frederick Jackson Turner. In a paper delivered before the American Historical Association in 1893, Turner argued that the frontier was the most important force in the making of a unique American character and society. From the first arrival of Europeans on the Atlantic coast, some were drawn westward by the lure of free "land." The result was an expanding line of settlement, the westward-moving boundary between "savagery" and "civilization." The challenges of the wilderness forced pioneers to adopt new values and ways of living, Turner wrote. From this social evolution came the traits he said were most distinctive to the emerging nation - its democracy, individualism, self-reliance, inventiveness. The frontier, in short, "explained" America, according to Turner; it was the cradle of the national character. And because his generation was witnessing the end of this settlement process, Turner was also raising the question of whether this admirable national character would change once more, perhaps for the worse, in the generations ahead.

Under the influence of Turner and his followers during the first few decades of the 20th century, the frontier was seen as more than a colorful chapter in the national past. It was the key to the shaping of much of American history. By the 1930s, however, critics were arguing that the "frontier thesis" overlooked many other factors in the making of a distinctive people. (Turner himself later had written that the rise of sections and regions was at least as important in explaining the national story.) Others complained that Turner's powerful but vague arguments could not support his sweeping claims.

Recent critics note that Turner neglected important figures, such as women and ethnic minorities, in frontier history. They also argue that the triumphal tone of the Turner school conceals the terrible cost exacted from people living on the land at the time the new arrivals arrived to take over. To scholars like Patricia Nelson Limerick, the term "conquest" more accurately describes the process of westward expansion. Events heroized in many histories and popular treatments have left a legacy of dispossession, poverty, exploitation, and resentment that remain a disturbing part of national life.

Part of that cost, for instance, has been a heritage of hostility with Mexico and nations to the south arising from the seizure of California and the southwest in 1846-48. This legacy is just one reminder, other historians would say, that there were many frontiers, many intrusions of cultures that were frequently in conflict. Earlier in the 20th century Herbert Eugene Bolton wrote on the importance of the Spanish "borderlands" and the significance of a Hispanic presence in the West. The continuing arrival of peoples and customs from Mexico and Latin America are reminders to many historians of the frontier's multinational inheritance, French and Russian as well as Hispanic. Recent scholarship has emphasized that the frontier was a mingling ground of many traditions, including Native American, that have survived in part until today.

Others have expanded that idea to include the study of frontiers elsewhere in the world where European nations established colonies and spread their cultural influence. In The Great Frontier, Walter Prescott Webb argued that Europe's expansion was the most important force in shaping the modern world's values and institutions. While few would go so far, historians have studied places as far-flung as South Africa, Latin America, and Australia to look for recurring patterns in the treatment and responses of native peoples, in economic development, in cultural life, and in conflicts between frontier settlers and their mother societies. The frontier in this context is seen as a complex global story.

Part of that global impact has to do with how people have imagined the frontier, not just with what actually happened. Images of free-ranging cowboys, noble Indians, courageous pioneer families, and slick gunslingers are familiar throughout the world. Depicted in novels and films, music, art, and other media, the mythic images made from historical episodes have been one of America's most successful exports. But for Americans, as historians such as Henry Nash Smith have written, this myth has had a special meaning. It expresses what they long to believe about themselves, the changing values, fears, and aspirations of a people explaining who they are.