The events you are referring to involve the Kansas-Nebraska Act of 1854, which was proposed by Senator Stephen A. Douglas. This act allowed the territories of Kansas and Nebraska to use popular sovereignty to determine whether they would permit slavery within their borders.
The act effectively repealed the Missouri Compromise of 1820, which had prohibited slavery in certain areas of the Louisiana Purchase. By allowing the settlers of these territories to decide the issue for themselves, the Kansas-Nebraska Act was intended as a compromise to ease tensions between pro-slavery and anti-slavery factions.
However, instead of delaying conflict, the act led to increased tensions and violence, particularly in Kansas, where pro-slavery and anti-slavery settlers clashed in a series of events that became known as "Bleeding Kansas." This violence showcased the deep divisions within the country over the issue of slavery and pushed the nation closer to civil war.