How did Stephen Douglas's plan undo the Missouri comproise?

Stephen Douglas's plan, particularly through the Kansas-Nebraska Act of 1854, effectively undid the Missouri Compromise by allowing the territories of Kansas and Nebraska to decide for themselves whether to allow slavery through the principle of popular sovereignty. This approach contradicted the Missouri Compromise, which had prohibited slavery in those territories north of the 36°30′ parallel. As a result, the act reignited sectional tensions and led to violent conflicts, known as "Bleeding Kansas," as pro-slavery and anti-slavery settlers rushed to influence the territories' decisions.