Abraham Lincoln said all Americans knew slavery "was somehow the cause of [the Civil] war."
The slave population in the U.S. was the only one in history that grew by its own means. This suggested some humanity towards them. States were less punitive than in other slave societies.
Slavery remained a burden to American democracy because it mocked its claim to be enlightened.
Thomas Jefferson was only one of many people who felt slavery conflicted with the principle of equality.
The early republic took steps to check its growth, including: 1787 Banning it in the Old Northwest; 1808 Banning the importation of slaves; 1820 Declaring the Missourri Compromise (Louisiana Purchase territories closed to slavery forever).
1790's - Eli Whitney's cotton gin sparked slavery again, because cotton became the cash crop, and it required a lot of work.
The lands won in the Mexican War raised the question of slavery in the west. 1846 - 1861 was filled with failed efforts to deal with it.
Some of these were: 1850 Compromise; 1854 - Kansas-Nebraska act; 1857 - Dred Scott (undid the Missourri Compromise).
Question was ultimately settled by the Civil War.
Southern rebels called the war "The War for Southern Independence." The war established the supremacy of the union.
After the war, weak Northern will and strons Southern resistence kept the blacks from becoming full American Citizens.
All the Civil War brought was freedom, though that was powerful.