Fugitive slave act
When slaves started to escape from plantation owners in the south and were not being returned it the south they got very angry. Because of that they made a new law called the fugitive slave law which forced you to return slaves. This was not at all successful because the north refused to return the slaves because they thought they should be free. This made the south mad that the law won't be enforced so they would not get slaves back and made the north mad that they are being told to return slaves.
Not only were they forced to return slaves but were also told to hunt down slaves and try to return them. People came up to the north from the south trying to capture slaves to return them and get the reward which was usually a lot of money.
When the fugitive slave law started being enforced people sided with the slaves in the nor and tried to protect them. This infuriated the south and caused them to retaliate by threatening seceding. Most people in the south at the time used slaves and it was a massive part of their economy so it was no surprise when they wanted to leave. The south thought they were not being treated fairly and wanted to be done with the north and their ideas about slaves being free. The north was mad because they thought they were losing rights by being forced to turn in slaves that ran away especially considering they saw how slaves were being treated and hated it usually.
Because of these reason the civil war started and the south was leaving. The north fought back to get their southern land and eventually the north came out victorious and won the war. The fugitive slave act was there to help the south but they thought it was not enough so they tried to leave. If the act was more enforced they probably would not have tried to leave and the war would not have happened.

://www.history.com/topics/black-history/fugitive-slave-acts