In the mid-19th century, the debate over slavery intensified significantly, affecting American politics and society. This era featured several key legislative measures and events that heightened sectional conflict. Among these was the Kansas-Nebraska Act, which allowed territories to decide the slavery issue through popular sovereignty. This act led to violent confrontations known as "Bleeding Kansas." Additionally, the Dred Scott decision by the Supreme Court ruled that African Americans could not claim U.S. citizenship, further inflaming tensions. As both the abolitionist movement and pro-slavery factions gained momentum, the question arose of whether the United States was ultimately headed towards Civil War. Which of the following actions is considered a catalyst that directly influenced the causes of the Civil War?