At the end of the Mexican War during Polk’s term as president, many new lands west of Texas were yielded to the United States, and the debate over the westward expansion of slavery was rekindled. Southern politicians and slave owners demanded that slavery be allowed in the West because they feared that a closed door would spell doom for their economy and way of life. Whig Northerners, however, believed that slavery should be banned from the new territories. Pennsylvanian congressman David Wilmot proposed such a ban in 1846, even before the conclusion of the war. Southerners were outraged over this Wilmot Proviso and blocked it before it could reach the Senate. When this act was denied it essentially caused America to become a country of two halves. Sadly, this division caused Americans to provoke wickedness against one another: the North vs. South, Slavery vs. Freedom, and Brother vs. Brother.
The Wilmot Proviso justified Southerners ' fears that the North had designs against slavery. They worried that if politicians in the North prevented slavery from expanding westward, then it was only a matter of time before they began attacking it in the South as well. As a result, Southerners in both parties flatly rejected the proviso. Such biased support was unprecedented and demonstrated just how serious the South really felt about the issue.
The large land concessions made to the U.S. in the 1848 Treaty of Guadalupe Hidalgo only exacerbated tensions between the North and the South. Debates in Congress grew so heated that even fist fights broke out between Northern and Southern politicians on the floor of the House of Representatives. In fact, sectional division became so evident that many historians label the Mexican-American War and the Wilmot Proviso the first battles that ignited the Civil War. Even though the Wilmot Proviso had failed, the expansion of slavery remained the most demanding issue in the world