The U.S. acquired two large pieces of land in the 1830’s and 1840’s where the issue of slavery was important and began to drive the northern section of the country apart from the southern section of the country. The land acquired was Texas and the Mexican Cession.
In 1836 Texas gained its independence from Mexico and immediately asked the U.S. for annexation to the U.S. The debate over annexation centered on a couple of issues, one being slavery. Texas allowed slavery and the debate over whether to annex Texas pitted northern interests that oppose slavery and annexation against southern interests who supported slavery and annexation. This debate lasted 9 years until 1845 when Texas was admitted to the Union as the twenty-eighth state, dividing north and south.
The annexation of Texas sparked the Mexican American War where an American victory led to the acquisition of the Mexican Cession, land that included all of California, Nevada and Utah, and parts of Arizona, New Mexico, Colorado, and Wyoming. The issue of slavery in the new acquisition led to the Compromise of 1850 which deepened the division between north and south. In the compromise, California was admitted as a free state and the slave trade was ended in the nation’s capital, Washington, D.C. These provisions upset the south. Two new territories, New Mexico and Utah, were established and slavery would be decided by popular sovereignty in these new territories. This meant that the people of the territory would decide the issue of slavery, which opened up a huge area to slavery. Also, a new fugitive slave law was passed. This mean that everyone, even people in the north, had to assist in returning runaway slaves. These provisions of the Compromise of 1850 upset people in the north. The Compromise of 1850 created tensions between the north and south, helping to drive the two sections apart.
The issue of slavery was the big deal that drove the two sides apart and the issue of expansion made it so that the previous compromises on the issue of slavery fell apart.
Slavery had been an issue (not really on moral grounds, but because it made the North and South have different societies and needs) for a long time. But the Compromise of 1820 had more or less settled the question.
Then came the Mexican-American War. This war got a huge new chunk of terrritory for the US and so the two sections had to argue once again about whether the new territory would be free or slave. This dispute drove the two sides farther apart and really led pretty directly to the Civil War.