When the Mexican-American War ended, the United States received the Mexican Cession which included all of California, Nevada and Utah, and parts of Arizona, New Mexico, Colorado, and Wyoming.  With this huge land acquisition, the issue of slavery in the new territories was raised.  This led directly to the Compromise of 1850.  This compromise deepened the division between the North and the South.  First, California would be admitted to the Union as a free state, which upset the South.  Two new territories would be established, Utah and New Mexico.  These territories would be open to slavery through popular sovereignty, that is, the people of the territories would decide the issue of slavery.  This upset the people in the North because so much territory could now have slavery. The slave trade was banned in the District of Columbia which upset the South because they saw this as a first step towards abolishing slavery.  There would be a new, strict, fugitive slave law which upset the North because now by law they had to assist in returning slaves to their owners. These issues were addressed in the Compromise of 1850, which was passed because of the results of the Mexican-American War and helped create the conditions for the Civil War.

The problem with taking so much territory from Mexico in the Mexican-American War was that no system existed to determine whether or not the new states would be slave or free.

The Compromise of 1850 reassured both the North and South in that it admitted one free state and one slave state, and the balance in Congress between pro and anti-slavery Senators and Representatives was maintained.  Secondly, both sides agreed that from that point on, popular sovereignty would decide the status of each new state. That means each state would vote yes or no on slavery and the majority would win.

Lastly, the South wanted to be reassured that rnuaway slaves were not being helped by abolitionists in the North, so the Fugitive Slave Act was included in the compromise, which made it illegal to aid runaway slaves in the North.