Turner's frontier thesis, perhaps the most famous theory in American history, argued that the closing of the American frontier in the 1890 census, which stated that there no longer was a frontier boundary line in the US, marked the end of the first phase of United States' history. More importantly, this thesis argued that the frontier was the single defining element in the development of the US spirit or character. Turner described this character, formed he argued by the existence of a frontier wilderness into which white Americans could throw their energies and talents, as one of

strength combined with acuteness and acquisitiveness ... practical inventive turn of mind ... restless, nervous energy ... [and] dominant individualism.

Without the outlet of the frontier for restless individuals, Turner worried about what the future of the country would hold.

Most historians now reject Turner's thesis that conquering the frontier was the master narrative defining the US experience up until 1890. Historians argue that we are defined by slavery, the huge and successive waves of immigration that populated this country largely with Europeans, and/or the rise of a powerful industrial machine in the last half of the nineteenth century. Others argue that the various "frontiers" in the American experience are so disparate that they don't cohere into a meaningful whole.

Many historians argue that it was community, not individualism, that made survival possible in the frontier. Almost all historians today also reject the triumphalist narrative that the spread of white culture represented "progress" over "savagery" and point to the genocide and suffering frontier expansion cost native peoples, as well as women and Asians brought to the frontier.

Turner's "Frontier Thesis" stated that westward expansion was important to the American psyche in that conquering these uninhabited lands made United States's citizens more self-reliant and industrious. Turner wrote this thesis in 1893, three years after the West was declared "settled" in 1890. Turner was concerned that by ending westward expansion, Americans would grow soft and lose their pioneer spirit. To Turner, this pioneer spirit and attitude made the United States into the growing commercial and industrial giant that it was becoming during the late 1890s.

While many historians and politicians such as Theodore Roosevelt became fans of Turner. later historians criticize the Turner thesis as being only concerned with the Anglo-Saxon view of American expansion. The Turner thesis does not take Mexican and Native American claims to the land into account. The Turner thesis can also be viewed as justification for imperialistic actions—by the end of the 1890's, the United States would expand its holdings beyond North America. Turner's took the view that the United States was morally justified in taking control of the West; this view has been criticized by recent historians as they view it as biased.

Turner was one of the most influential historians of his time; however, his thesis concerning the American West has been criticized by most modern historians for its bias.

Frederick Jackson Turner produced the "Turner Thesis" in 1893 shortly after the 1890 Census had determined that the American frontier had closed. Jackson argued that the frontier was a vital part of American democracy, as it allowed for continual reinvention and for limitless new opportunities for Americans (by which he meant white Americans) to achieve what he defined as progress. His thesis helped instigate an interest in American imperialism so that Americans would have access to new opportunities and markets abroad after the closing of the American frontier.

Many recent Western historians, such as Patricia Nelson Limerick in her Legacy of Conquest, published in 1987, questioned Turner's narrative of celebratory white progress on the frontier (see the source below). Instead, she looked at the experience of other groups on the frontier, including Mexicans, Native Americans, and Asians, and at their exploitation and suffering at the hands of white western pioneers. She argued that these groups were pushed aside by white settlers. Historians have questioned whether the white western movement represented unalloyed progress or instead imposed suffering and limitations on other groups.

In his paper "The Significance of the Frontier in American History," presented in 1893 in Chicago, Frederick Jackson Turner glorified westward expansion as demonstrating the virtues of the American spirit. This American spirit consisted of crossing continents and winning wilderness. He outlined the importance of civilization meeting savagery and emphasized the strength, power, and energy of the settlers who traveled westward. He also proposed that there were three waves to settling the West. First, the pioneers built the cabins and tamed the land. Next, the emigrants purchased the land, forming towns resulting in democracy and civilization. Lastly, men of capital and enterprise arrived and developed towns into bustling cities. Turner also discussed the importance of free land and equated this free land with opportunity.

Critics of this paper emphasize that this "free land" was not free. Prior to westward expansion, Native Americans, Hispanics, and mixed race people inhabited the West and had their own cultures. Instead of being "settled," critics argue the land was conquered, forcing many peoples out of their homes and into reservations. The result of this wave of expansion also resulted in oppression and greed. Towns sprang up as people flooded the West to search for gold. Later, these settlements became ghost towns. Instead of Turner's view of westward expansion as glorious, critics believe it inglorious and corrupt.

Turner argued that the frontier impacted the development of American democracy. Americans have an independent spirit characterized by self-reliance, strength, ingenuity and practicality. These traits were the basis of American culture and democracy. He was the frontier as an outlet for the restless American spirit.  Many critics feel the development of the uniquely American character and democracy is much more complex and cannot be explained so simply. Additionally settlers in the West often relied on others to be successful - wagon trains, barn raisings etc. Conquering the West was far from the romantic ideal seen in literature and movies. While this may be seen as a great opportunity for white settlers, it involved the destruction of Native American culture.