The document discusses how the United States saw itself as two countries in the early 19th century - the North and South had vastly different economic systems and ways of life centered around slavery, placing tremendous strain on the nation. It led to the Civil War as tensions escalated over several decades and the South felt its economic livelihood and way of life were being threatened by the North's growing opposition to slavery. This divide showed that one nation can contain two very different societies that are incompatible in the long run if left unresolved.