What is the ""unfinished"" work or ""great task remaining before"" the american people?

They have to finish what they started: Win the war, unite as a Union and end slavery.

Explanation: The American civil war was a war that started in the year 1861, it took place as a result of tension about the abolition of Slavery, the war took place between the Northern American states and the Southern American States. In the 19th century the Northern American states were very prosperous as they were more industrialised with many industries while the southern states were mainly filled with farmers who do large scale farmers, the southern states don't want the Slavery to be stopped while the Northern States are working for the abolition of the slave trade