Answer:
Slavery divided the nation so deeply during the 1800s, that the issue of slavery led to the outbreak of the American Civil War, the deadliest war in American history.
Step-by-step explanation:
The reason is that the Constitution of the U.S. did not resolve the issue of slavery, and allowed slavery to continue existing in the Southern half of the country, where the economy was based on slave labor.
As decades passed, more and more people became aware of the horror of slavery, and launched an abolitionist movement that had political representation. When this movement gained more and more political power, the slave states of South became suspicious, which led to their final secession when Abraham Lincoln, a moderate abolitionist, gained presidential power in 1860.