The main way in which geographic and economic differences between regions led to increased sectionalism in the US was that the South was entirely agrarian and depended heavily on slavery, while the North despised the institution of slavery. This eventually led to the Civil War.
They moved to the city so they would have an easier time finding work
The idea of manifest destiny greatly helped shape the U.S. governments's policies of land acquisition, since this held that the US was "destined" to expand from the Atlantic Ocean to the Pacific--meaning that the US embarked on a series of wars with Mexico and confrontations with Natives that helped the settlers move west.
