Final answer:
The Mexican-American War led to the United States' acquisition of new territories, which intensified the debate over the expansion of slavery and increased sectional tensions that contributed to the onset of the Civil War.
Step-by-step explanation:
The Mexican-American War and Its Impact on Sectional Conflict
The war that resulted in territorial gains for the United States and caused sectional conflict in the years leading up to the Civil War was the Mexican-American War. Following the war, the Treaty of Guadalupe Hidalgo was signed in 1848, which led to the United States acquiring a significant amount of territory in the West. This acquisition brought to the forefront the contentious issue of whether to allow the expansion of slavery into new territories such as California and New Mexico. The dispute over this question heightened tensions between the North and South, particularly regarding the economic and political implications of slavery.
National political leaders attempted to address these sectional tensions with the Compromise of 1850. Despite efforts to maintain peace, subsequent events like the Fugitive Slave Act, Kansas-Nebraska Act, and the Dred Scott decision only deepened the division between the North and South. These growing tensions culminated in the election of Abraham Lincoln in 1860, which resulted in the secession of several Southern states and set the stage for the Civil War.