The Westward expansion
From 1861-1865 a bloody war between the Northern States and the Southern States took place. The Northern States were called the Union and the Southern States were called the Confederacy.
The dispute between the Northern and Southern States was primarily about different production- and life forms. The North had a more industrialized society. The South was a largely agricultural region, dominated by large cotton plantations of slave operation.
The South not only struggled to preserve slavery in the South, which was formed after The American Revolutinary War. They also fought for slavery should be allowed in the new areas of The United States. The Northerners did not accept this. They wanted the new areas should be free of slaves.
The disagreement between North and South ended with several Southern States withdrew from the Union and formed the Confederacy. This happened in 1860. The same year, Abraham Lincoln was elected as President. He was against the extension of slavery to the new states. Therefore, in 1860-1861 the Confederacy withdraw from the Union, as a protest.
Northerners considered the Confederate withdrawal from the Union as a rebellion act against the entire United States.