The Mexican American War from 1846 to 1848 was a victory for the United States over Mexico. With this victory, the American borders greatly expanded. The aftermath of the Mexican American War led to increased political tensions between the North and South over the expansion of slavery into the newly acquired territories from Mexico. To resolve brewing tensions, Congress passed the Compromise of 1850 and allowed popular sovereignty in the territories, later giving rise to further inflamed conflicts. As the Mexican American War concluded, regional tensions gradually propelled the two regions into Civil War. 

After the treaty of Guadalupe-Hidalgo, Mexico ceded half of its territories to the United States. This ceding, known as the Mexican Cession, more than doubled the size of the United States but it also led to political dissensions on how the newly acquired territories should attain statehood as either a slave or free state. Politicians at the time were very responsive to the issue. For instance, the Free-Soil Party emerged to fight against slavery and the Republican Party gained prominence with the support of abolitionists and the common citizens. Of course, during this time by the 1850s, Democrats in the South were getting ever distrustful of the North, especially with the Northern attempt to pass the Wilmot Proviso (which aimed to contain slavery where it is and prevent its spread to new territories). The contentions eventually led to another compromise that temporarily delayed the Civil War. 

As the North and South became embroiled over political party lines,  Henry Clay, the Great Compromiser, once again stepped in and created the Compromise of 1850, attempting to appease both sides. Within the Compromise, it allowed California to enter the Union as a free state, further allowing Western territories (when applying for statehood) to decide for themselves whether or not to have slavery, settled Texas’s claim over New Mexico, abolished the Slave Trade in Washington D.C., and allowed a harsher Fugitive Slave Act. After the Compromise, it is evident that the North made the largest gains. However, the South was still placated with a more stringent Fugitive Slave Act since it allowed slavecatchers to go North and request the help of Northerners to help return escaped slaves.  In the end, it would be the Fugitive Slave Act of 1850 that pulled the two regions apart. Northerners, who once before may have been passive bystanders of slavery were now obligated by law to assist slavecatchers, this infuriated many Northerners. In fact, a novel of the time period - Uncle Tom’s Cabin by Harriet Beecher Stowe - exemplified to the North that Southern slavery was a great evil. Stowe’s novel greatly offended Southerners, further convincing them that the only way to protect their livelihood was through secession. 

Another key component that increased regional tensions would be the policy of popular sovereignty in the new territories. For instance, the Kansas-Nebraska Act of 1854 turned the territories above the line of freedom (based off of the Missouri Compromise of 1820) into new territories open to slavery. This issue came into being with Stephen Douglas wanting to build a railroad through the territories to Illinois. In order to get the necessary votes from the South, he proposed popular sovereignty in the territories above the Missouri Compromise Line and proclaimed that whether or not a state wants slavery is totally dependent upon what its residents desired. This compromise worked for the short term; however, this Act soon gave rise to violent conflicts between settlers of the territories who had divided views on slavery. One example would be Bloody Kansas, in which John Brown led an insurrection on whether Kansas should be admitted to the Union as a free or slave state with the Pro-Slavery settlers. This attack by Brown further convinced Southerners that the North does not care about their interests and that all Northerners wanted to get rid of slavery. One event that officially confirmed the Southerners' view would be John Brown’s Harpers Ferry raid in western Virginia (present day West Virginia) where he attempted to seize a federal arsenal and incite a slave rebellion. Brown’s raid failed but his actions sowed seeds of secession and hostility in Southerners.  

Incontrovertibly, many factors contributed to sectional tensions between the North and South, such collision of interests steadily culminated into the Civil War. The spread of slavery, the Compromise of 1850, and bloody conflicts from extremists all thrusted the two regions into a deep abyss of distrust. In an important way, the American victory of the Mexican American War highlighted protruding sectional tensions and caused what was a delicate balance of free and slave states to crash under the acquisition of land cessions.