let me guess you got this out of a history book. if you did then the answer is manifest destiny which was the american peoples dream to have the US span all the way from"sea to shining sea" so basically california to viginia if you did not get this from a history book then the answers are endless and can be anything
A myth. But using the term "myth" I don't mean it in a disparaging way. Myths are what give many people's lives purpose and/or direction. The myth usually conjures up a nice house in the suburbs with a two car garage housing a family of 2.5 children and perhaps a nice white picket fence surrounding the property. I suspect the only time the myth was closest to absolute reality was the decades immediately following WW2. When dad was the sole bread winner and many mothers stayed at home raising the kids.