Westward expansion was born out of the desire by the US to expand its territory further hence the concept; Manifest Destiny. American government believed that it was entitled to exercise its powers all over the world. In order to do this, it needed to have more states, thereby necessitating the desire for conquest. As the US gained more territories, it sent her people to occupy those states. This movement did not involve the white Americans only, but they were accompanied by their slaves. For example, the movement and subsequent occupation of Texas required laborers to work in the plantation fields, the occupation of California required laborers to assist in the mining of gold that was discovered, and when they occupied Oklahoma, the question of slavery was in everyone’s mouth. This was the relationship between the westward expansion and slavery. That is, as America expanded, the demand for slaves went up due to the increased land for cultivation and other economic activities.
The Mexican-American war fought between 1846 and 1848 further contributed to the sectional conflicts and the slavery debate. As America acquired more territory in Mexico, a bill was placed in congress forbidding any further extension of slavery to the newly acquired states of Mexico. The bill received support from the northerners but was opposed by the southerners who wanted slavery to spread to these states thereby, causing a major rift between the two. Wilmot Proviso, as it came to be known, threaten to tear the country apart as senator John C. Calhoun argued that the free slave state and slave state would be divided. This tension heightened to the extent that it became a foreign policy and escalated with the election of Abraham Lincoln as US president, who supported Wilmot Proviso. This led to the secession of some southern states to form a confederate, and further tension increased that later led to the battle of Antietam.
The conflict between the slave state and free slave state was meant to occur irrespective of whether America acquired more states or not. This was basically because the level of awareness and the dawn of a new world had begun. That is a world that valued humanity, freedom, and equality. Therefore, at one moment in time, slavery had to be stopped, and the only way was to engage in a physical war.