Civil war argument paper topic

The topic I chose is why the union should fight the Confederacy to return them to the union. I think my central argument will be about how small of a population they have, how it is a lot of enslaved African-Americans, and how easy it would be to keep them in the union. Even when slaves do join the war, the union will still outnumber the Confederacy in population and army size. I don't know why the union wouldn't try to force them to rejoin and stay in the union when it would be so easy to do so. Those are some of the things that I will include in my essay,