Which historian argued that the continuous existence of the American frontier shaped the character of the nation?

The historian who argued that the continuous existence of the American frontier shaped the character of the nation is Fredrick Jackson Turner. In his influential "Frontier Thesis," presented in 1893, Turner posited that the frontier experience was fundamental in developing a distinctly American identity. He believed that the challenges and opportunities presented by the frontier fostered traits such as individualism, democracy, and innovation. Turner's thesis suggested that the process of westward expansion was crucial in shaping American culture and social dynamics, effectively linking the notion of the frontier to the broader narrative of American exceptionalism.

In contrast, other historians mentioned may have different focuses; for instance, Charles Beard was known for his economic interpretation of history, David McCullough often writes about specific historical figures and events rather than broad societal themes, and Henry Adams is widely recognized for his writings on history and politics but didn't emphasize the concept of the frontier in the same way. Thus, Fredrick Jackson Turner stands out for his unique and impactful contribution to our understanding of how the frontier influenced the national character.