What was an underlying source of tension in westward expansion during the 1850s
One of the underlying sources of tension in westward expansion during the 1850s was the issue of slavery. The United States was sharply divided between free states and slave states, and the expansion of territories into the West brought this division into question.
With the acquisition of new territories like the Mexican Cession (1848) and the Gadsden Purchase (1853), the question of whether these new lands would allow slavery or be free became a key point of contention. The Compromise of 1850 attempted to address this issue by admitting California as a free state and allowing popular sovereignty in other territories, but it only temporarily eased tensions.
Another factor contributing to the tension was the conflict over the Kansas-Nebraska Act of 1854. This act organized the territories of Kansas and Nebraska and allowed settlers to decide whether to allow slavery through popular sovereignty. The act effectively repealed the Missouri Compromise of 1820, which prohibited slavery in the region north of the 36°30' parallel.
The passage of the Kansas-Nebraska Act led to intense rivalries and conflicts between pro-slavery and anti-slavery settlers in Kansas, resulting in violent episodes such as "Bleeding Kansas," where both sides clashed over the outcome of slavery in the territory. This further escalated tensions and contributed to the growing divisiveness between the North and the South.
Ultimately, these underlying tensions surrounding the issue of slavery in newly acquired territories played a significant role in the lead up to the American Civil War, with the 1850s serving as a critical period in the intensification of these tensions.