You may be aware that while the American Civil War began as a political struggle over secession (i.e. the preservation of the nation), it also had religious overtones. This was due in part to some misguided and/or self-serving influential southern preachers. Thankfully, northern abolitionists, mostly Christians, preached the good news of liberation for slaves. Harriet Tubman, Abraham Lincoln, and many lesser-known Christians spoke out against slavery and pointed out the true way of Christ.
Keep in mind, Christians had a history of slavery reform long before the Civil War. For example, as early as 1688, the Religious Society of Friends,
opposed slavery and protested against it.