Why was it difficult to reunite the north and south?

it was difficult to unite them because The north was against slavery( anti-slavery) while the south encouraged slavery

Explanation:

The main division between the North and the South was mainly because of slavery. the southern states encouraged slavery because they saw slaves as legally acquired property that should be kept and used at will. like in the cultivation of their plantation and also to be forced for indecent and dehumanizing jobs. while the North kicked against slavery through the Abolition movement which has been on even before the American civil war. slavery saw so  many Native Americans as well as Africans been transported through the trans Atlantic slave route. one the Northern states that rejected slavery from the onset was the colony of Georgia.

The southern states encouraged slave Trade because they saw it as been economically viable .

the treaty of brest-litovsk was a peace treaty signed on 3 march 1918 between the new bolshevik government of russia and the central powers (german empire, austria-hungary, bulgaria, and the ottoman empire), that ended russia's participation in world war i.