The right answer is the Compromise of 1850, back in 1850 the US was divided in two big fronts because the southern states supported slavery, while the northern states didn't. The compromise of 1850 was a pack of five laws that intended to stop the conflict between north and south, in this compromise California is admitted as a free state, Texas left its claim of attaching New Mexico to its territory and, as the question mentions, the most controversial element was the Fugitive Slave Act, which stated that fugitive slaves had to be returned to their owner, this was intended to stop the escape of slaves to the north and the economic loses that the south faced because of it.
