Final answer:
Frederick Jackson Turner's Frontier Thesis is considered deeply flawed because it oversimplified the development of American identity, neglected the role of Native Americans and various social institutions in the West, and held a mythic view of the frontiersman.
Step-by-step explanation:
Later historians have viewed Frederick Jackson Turner's Frontier Thesis as deeply flawed for several reasons. Turner's thesis suggested that the American frontier was the defining factor of American democracy and character. It construed the West as a 'meeting point between savagery and civilization' where American society was constantly reinvented. However, Turner's approach is seen as a gross mischaracterization of the West because it largely ignored the role and agency of Native Americans, did not adequately address the impact of families, communities, government, and commerce in the development of the West, and subscribed to a mythic and individualistic view of the frontiersman.
Turner argued that the frontier served as a 'safety valve' for Europe's overpopulation and presented a unique environment for the shaping of American identity. However, modern historians critique Turner for his narrow focus and his omission of how Western expansion also involved significant violence, displacement, and the suppression of indigenous populations. In summary, later historians recognize that the historical complexity of the West was much broader than what Turner's thesis accounted for, challenging the notion that the frontier was the sole or primary engine of American democracy and identity.