The war that involved a conflict over newly acquired territories following the Mexican-American War was the American Civil War. After the Mexican-American War concluded in 1848, the United States acquired vast territories, including California and New Mexico, which intensified debates over the expansion of slavery into these new lands. These tensions eventually contributed to the outbreak of the Civil War in 1861, as Northern states sought to prevent the spread of slavery, while Southern states wanted to protect and expand it. The clash over these newly acquired territories was a significant factor in the growing sectional divide that led to the conflict.