Answer:
Fredrick Jackson Turner argued about the importance of the western frontier in American history in 1893:
He thought that the frontier made United Stated different from Europe.
He argued that the identity and vitality of America was due to its frontier.
He argued that American democracy depended on it.
Step-by-step explanation:
He even wrote a thesis on "The Significance of the Frontier in American History" in which he argued that the frontier would help America keep it´s democracy, this means keep it´s soberanity. The control of who enters and who doesn´t.
He also shared this ideas that the expansion on the west would change American culture and views.