Which is the BEST description of the historical impact of James K. Polk's presidency?

James K. Polk's presidency (1845-1849) is historically significant for its expansionist policies, which culminated in the acquisition of vast territories through the Mexican-American War and the Oregon Treaty. His administration fulfilled the idea of Manifest Destiny, promoting the belief that the U.S. was destined to expand across the continent. This territorial expansion, however, intensified debates over slavery in the newly acquired lands, contributing to the sectional tensions that eventually led to the Civil War. Overall, Polk's presidency reshaped the nation's borders and set the stage for future conflicts over slavery and statehood.