Which political movement aimed to prevent the expansion of slavery into newly acquired territories?

The Free-Soil Movement emerged in the mid-19th century as a response to the contentious issue of slavery in the wake of land acquired from the Mexican-American War. Central to the movement was the belief that slavery should not expand into the newly acquired territories in the West. It comprised a coalition of various groups, including abolitionists, former Whigs, and Democrats who opposed the extension of slavery, advocating for "free soil, free speech, free labor, and free men."

This movement set the stage for intense debates over the status of slavery in the territories and contributed to the rise of subsequent political entities, including the Republican Party, which later adopted a more organized platform against the spread of slavery. The movement specifically targeted the idea that new lands should be reserved for free settlers rather than for slaveholders, thus creating a significant political and social push against the institution of slavery during this period.