I am certain that the answer is the Kansas-Nebraska Act
The Kansas-Nebraska Act replaced the Missouri Compromise of 1850, which basically stated that for every free state submitted, a slave state would be submitted as well to keep the balance of slave and free states.
The Kansas-Nebraska Act replaced that by stating that slavery in all of the new territories would be decided by popular sovereignty, which is simply put as, decided by the people.
I hope this helped!! Have an amazing day :))
