This textbook points out that the US Constitution made no provisions concerning the expansion or limitation of slavery. As new territories were added to the country, debates as to whether slavery would be permitted erupted. Naturally, free-soilers wanted to limit the expansion of slavery, while those in the pro-slavery camp wanted it to expand. After the Mexican-American War, some people proposed that it should be left up to popular sovereignty as to whether slavery would expand into a newly acquired territory. This went against the Missouri Compromise of 1820 which prohibited slavery in north of the 36° 30´ latitude line

The issue of popular sovereignty came up in force in 1854 with the Kansas-Nebraska Act. Senator Stephen Douglas needed Southern support to organize these territories. However, they lay North of the demarcation line established in 1820. He knew that Southerners would only support the territorial organization if there was a chance that slavery could be established there. Therefore, popular sovereignty was written into the bill. By overtly favoring pro-slavers at the expense of free-soilers, this new act led to bitter tensions between Northerners and Southerners. When the Whigs failed to stop its passage, their entire political party crumbled. From its ashes, the new Republican Party was formed. With a platform that was overtly against the expansion of slavery, many Southerners felt their way of life to be under threat. This would eventually lead to their secession when Lincoln, a Republican, was elected president.

Even before secession, pro-slavers and free-soilers flocked to these territories. They each wanted to outnumber the other group in order to establish the territories as either free or with slavery. Things got so heated that violence erupted. What became known as "Bleeding Kansas" foretold the violence that would erupt across the country in the 1860s.