I guess more specifially I am trying to understand the differences between the Turnerian theories and the New Western History theories.
Frederick Jackson Turner, most well known for Turner's Thesis, suggested in the late 19th century that the frontier was the defining element of American culture, in that it reinforced the sense of individualism and self-reliance on which Americans pride themselves, contributed to our gun culture (which is very difficult for the rest of the world to understand) and that the resources that accompanied the frontier's settlement were the engine and origination of our economic success. He also said that the frontier, for all intents and purposes, was over and done with by the final 1890 conquest of the Native Americans in the Dakotas.
Limerick, on the other hand, as a modern historian first published in the 1980s with her most acclaimed and criticized book, The Legacy of Conquest, looked back on Turner's conclusions and suggested a little revisionism was in order. She tears away any romantic notions Turner might have had about the frontier, and suggests that in addition to the economic benefits that accompanied the move west, the frontier also developed the worst and most maligned aspects of American business and personal culture and history - the annihilation of the Native populations, the exploitation of the land and immigrants from both Europe but especially China for immense personal wealth in the hands of a few, and that the frontier turned America into an empire, to her long term detriment.
She doesn't disagree with Turner completely, nor does she denigrate him as a historian, she simply has the benefit of hindsight while Turner himself was witnessing events as they unfolded, unable to see what long term problems such a conquest might bring with it.