Answer:
The Kansas-Nebraska Act of 1854 was a piece of legislation that greatly exacerbated tensions between the North and South and contributed to the start of the Civil War. Here are some key points:
• The Act was sponsored by Illinois Senator Stephen Douglas and organized the territories of Kansas and Nebraska. It allowed the residents of those territories to decide for themselves whether to allow slavery.
• This introduced the concept of "popular sovereignty," which went against the Missouri Compromise of 1820 that had prohibited slavery north of latitude 36°30'. The Act effectively repealed this critical compromise.
• This created a firestorm of controversy. Northerners were outraged that a longstanding slave restriction had been overridden. Southerners saw it as opening up new territory where slavery could potentially expand.
• The Act resulted in massive immigration to Kansas by both free-soilers who wanted to keep it a free state and pro-slavery advocates, worsening tensions and eventually sparking violent conflicts known as "Bleeding Kansas."
• The controversy deeply divided the national political leadership and inflamed sectional tensions between the North and South. Former allies like Douglas and Abraham Lincoln became bitter rivals.
• It ruptured the delicate political balance between slave and free states, increased sectionalism, hardened attitudes on both sides, and thus played a major role in causing the circumstances that would eventually lead the nation into civil war.
So yes, an historian could argue that the Kansas-Nebraska Act - by overturning the Missouri Compromise, sparking violent conflict, and further dividing the nation along sectional lines - represented one of the most consequential events in pushing the United States towards disunion and civil war in the decade prior to the first shots at Fort Sumter.