The young country grew quickly, and by the middle of the nineteenth century it had 31 million people. But there were serious differences between the North and the South And in 1861 a terrible war started. At least 600,000 people died in the battles or from disease. The war began because the southern states kept slaves to work in the cotton fields but slaves were not allowed in the North. People from the South wanted to have slaves in the new lands of the west, but people from the North argued against this. In 1860, Abraham Lincoln was elected sixteenth president of the USA. He belonged to the Republican Party,