The Kansas-Nebraska Act of 1854 is highly significant, especially in terms of the question of slavery. However, first it allowed the territories in question to split into two states: Kansas and Nebraska. This opened new lands for settlement. The people in these new territories could, more importantly, decide for themselves whether they would allow slavery within their boundaries. (Originally there was a business angle as well, with the hope to introduce the railroad into the territory.)
The act also ended the Missouri Compromise which had been passed in Congress in 1821 by pro-slavery and anti-slavery factions which attempted to control slavery in the West, and other areas in the U.S.
Stephen Douglas, a Democrat of Illinois, drafted the Kansas-Nebraska Act; it allowed the states in the North to decide to end slavery within its states, while the South could still allow slavery in that region. The act soon came under criticism as a tool to placate the "slave powers" of the South.
It was at this point that the new Republican Party was formed to oppose the act and stop the spread of slavery in the United States. It became a strong political party in the North.
First, what was the Kansas-Nebraska Act? In 1854 the question of what to do with all the unorganized lands that the United States owned. Would slavery be allowed in those lands or not? How to decide...
Stephen Douglas, the Illinois Senator, came up with what was to be known as the "Kansas-Nebraska Act." This law would create the territories of Kansas and Nebraska and allow the people who moved there to vote on whether they wanted slavery or not. Sounds like a good compromise, but in the end it didn't work out so hot.
Both sides of the slavery debate (abolitionists and pro slavery) rushed to move people into the states to win the vote. Pro Slavery people largely came from Missouri while Anti-slavery people came from further East. It was such a hot issue that the two groups came into bloody conflict with one-another. At one point, two separate governments were set up in different towns (one free, one slavery.) There's a reason they called it "Bleeding Kansas."
In one instance, an abolitionist named John Brown "captured" five pro-slavery farmers and murdered them in retaliation for murders committed by pro-slavery forces.
In the end, Kansas was admitted to the Union as a Free State, but the turmoil caused by the law actually helped further fracture the nation and propelled the country toward civil war. it brought to a head the bitter divide that was cracking the country in two.
Kansas-Nebraska Act of 1854 repealed the Missouri Compromise of 1820. It applied the principle of popular sovereignty to the territories. It permitted the expansion of slavery beyond the Southern states. It also sparked the formation of the Republican party. This Act would be one of the causes of the Bleeding Kansas and the Civil War.