Over the next 150 years the country experienced massive growth through purchase (Louisiana and Alaska) and war (much of the American West and southwest from Mexico). However the social atmosphere in the country also continued to change and fights over slavery and state versus federal rights eventually led to the American Civil War (1861-1865), which was won by the north and ended slavery, but not segregation or racism.
After the Civil War, the U.S.A. continued on its path of westward expansion and faced further social growing pains with huge immigration by Europeans fleeing political instability or famines, and the former slave population moving from the south to all parts of the country. With this new labor pool and the help of the industrial revolution, savvy businessmen used these immigrants as cheap labor, which led to a series of legal changes to protect the worker, improving the quality of life, healthcare, communication, and transportation networks.
WWI and WWII forced the U.S.A. onto the international stage despite their best efforts to remain a hermit. This period proved the U.S.A.'s power on the international stage and soon the country because a world power and a leader in innovation from mass production of the car and the production of the airplane to exploiting the usefulness of mass communication networks like television and movies.
Today the U.S.A. continues to change and grow as migration has repeatedly shifted: from abroad to the U.S.A., then from rural to urban, and finally to suburbia, in which nearly every family has their own house, land, and car.