Final answer:
The Kansas-Nebraska Act led to 'Bleeding Kansas' and violence due to conflicts over slavery. John Brown, an abolitionist, intensified these clashes with acts such as the Pottawatomie Massacre.
Step-by-step explanation:
The Kansas-Nebraska Act led to violence because it allowed the territories to decide on slavery via popular sovereignty, igniting conflict between pro-slavery and anti-slavery settlers. John Brown was a radical abolitionist whose violent actions, such as the Pottawatomie Massacre, escalated tensions and undermined the ideal of peaceful popular sovereignty.
These events contributed to the period known as Bleeding Kansas, marked by significant bloodshed, and foreshadowing the Civil War.