Willa Cather's O Pioneers! says a great deal about the trials and tribulations of a family struggling to make their way in the unfamiliar environment of the Great Plains. Alexandra, the protagonist, embodies this struggle, and in many ways she internalizes it to the point that she comes to resemble the land itself. In addition, one can argue that Alexandra's difficulties relating to her survival mirror those same diffiiculties of America itself.
America is the land of opportunity, and one can make something of opportunities if provided with them. In the early years of Colonial America, the individual colonies endured a number of hardships during their existence as satelllites to the English crown. In the same way, Alexandra finds herself presented with numerous hardships, many of them resulting from the nature of the environment itself. Her environment defines her existence, and she is beholden to it. This tie of dependence between Alexandra and the land directly mirrors the dependence of the colonies on England in their early history.
Alexandra also resembles America in the sense that her success is determined by seizing opportunities when they arise. When Alexandra finds a way that she can make farming on her land more effective, she takes that opportunity, providing her with a chance of success. In the same way, as the colonies approached what certainly looked like certain revolution against England, they had to endure the greatest of hardships as a result of the various prohibitory acts brought against them. When they saw an opportunity to break from England's control, they took the opportunity to do so. Alexandra's struggle for her independence culminates in her ability to do things on her terms, and the same can be said for America after the end of the American Revolution.