Final answer:
The territory that became a battleground between proslavery and anti-slavery forces in the 1850s is Kansas, particularly after the Kansas-Nebraska Act of 1854, leading to the violent period known as 'Bleeding Kansas.'
Step-by-step explanation:
The territory that became a battleground between proslavery and anti-slavery forces in the 1850s is Kansas. Following the passage of the Kansas-Nebraska Act in 1854, which applied the principle of popular sovereignty to the region, both proslavery and antislavery advocates flooded into Kansas in an attempt to influence whether it would become a slave state or a free state. This conflict rapidly escalated into violence, referred to as 'Bleeding Kansas,' as both sides engaged in fierce confrontations with each other, providing a stark demonstration of the growing sectional tensions that would eventually lead to the American Civil War. Notably, these events directly challenged the previous balance established by the Missouri Compromise, and the violence and political strife served to ignite and define the volatile issue of slavery's expansion into America's newly acquired territories.