I think that the cause of the civil war was the argument with slavery and it got even worst when the western part the United States were earned. The Abolitionist in the North did not want slavery anywhere and the Southern Planters (Slaver Holder) said that slavery was fine anywhere even in the west. If you thought the United States was divided only during the Civil War, you are wrong. The slavery issue divided the nation, and the nation was also divided by new political parties, and different views.
One problem that caused the Civil War was the Balance of Free States (Slavery was banned in these states) and slave states (states that allowed slavery) and if the number of states were even that meant that there would be an even amount of slave state representatives in Congress as the free states. If you didn't know America's fifty states didn't come with us for England we had to form them. So back in the day in 1819 there were 22 states, eleven free states and eleven slave states. Then Missouri wanted to be a slave state because it was only a territory. Well that meant there would be 12 slave states and 11 free states and the balance of free and slave states would be disturbed. Then Senator Henry Clay had an idea. He said since Maine wants to become a free state then it is O.K. for Missouri to become a slave state. This was called the Missouri Compromise. There was some else to that compromise and that was the Missouri Compromise line and this line was at latitude 36°30' N (In English this means that the line is at the southern border of Missouri) and it only applied to the Louisiana Purchase. For the Louisiana Purchase any land above the line, slavery would be banned, and any land below the line slavery would be allowed. If you go further into history you see that this didn't help and the only thing congress was doing was making more and more compromises like the compromise of 1850, the Kansas-Nebraska Act, The slave state and free state problem divided the nation and problems like these didn't stop until the Civil War started.
The United States was divided before the Civil War by the different views of slavery. First, there were Abolitionist and they thought that slavery was morally wrong and should be abolished (hence the name "Abolitionist"). Southern slave holders said slavery should be allowed anywhere and it isn't wrong. Moderates had two different views. One moderate could say that the Missouri should extend all the way to the western border of the United States. another moderate view was let the people who live in that area choose if they want slavery where they live, giving them popular sovereignty. As you can see because of slavery the Nation was divided by the different views of Americans.
The Nation was also divided by the new political parties that were formed at that time. At that time the political parties were Northern Democrats, Southern Democrats, and Whigs. The new parties that were formed were the Free-soil party and the Republican Party. First, I'll tell you about the free-soil party. The Free-soil Party was formed in 1848 and it was just made of Northern Democrats and Whigs. The main goal of this party was to keep slavery out of the western territories. In the mid-1850 a new political party is form again. The Republican party. This party was made up of Free-Soilers, Whigs, and Northern Democrats. There main goal was to keep slavery out of the Western Territories. I guess you see that all these arguments a war was the only way to solve this.
Before the Civil War the slavery issue made people start fighting each other. There was fighting in the south, where mobs would attack abolitionist like William Lloyd Garrison, who was the editor of the Antislavery newspaper, The Liberator. There was fighting in Kansas, where popular sovereignty was being tested but it didn't work. Kansas ending up having two governments one wanting slavery, the other wanting no slavery. Also there was fighting in Congress, verbally and psychically too. People where theatrening to shoot each other in Congress. I guess the only way America could solve this problem was with a war.