Lincoln suggests that the Civil War...

Slavery, Lincoln stated, was the reason for the war.

Explanation:

Lincoln stated, was the reason for the war is slavery . Referring to the slaves as “one-eighth of the whole population” suggested that they were part of the nation, not an exotic, unassimilable element, as he had once viewed them.

it is the background information on the characters and setting explained at the beginning of the story. the exposition will often have information about events that happened before the story began. the exposition is often the very first part of the plot.

the underlined phrase is an example of : understatment