1)Slavery:this is the main reason that lead to the war, southern usa invested in slaves(blacks) to work as laborers in fields. the northern usa saw slavery as evil and sought to end it
2)Abraham Lincoln: the final straw for the south was the election of lincoln as the usa president because he was the member of the anti-slavery republican party. lincoln helped form the war that ended slavery in usa.
3)Seccession: when lincoln was elected, many southern states wanted to secceed from usa and form a new country called "confederate state of america". abraham lincoln said the have no right to leave and sent troops to invade the southern states.
4)expansion: the usa was expanding westward and new states were added and the north and south were battling for powers.
5)industry and farming:in the mid 1800s, the economy of the northern state has moved from farming to industry and people lived in big cities like new york, boston, etc.
Note: The north(union) won the war against the south(confederacy) which ended slavery.