In the years before the Civil War began, there were growing sectional differences between the North and the South, including constitutional arguments over states' rights and the Federal Union; economic disputes, due to industrialization in the North and agriculture in the South, over issues such as tariffs and internal improvements; and the larger issue of slavery, which the South was in defense of. For example, after the Mexican War, the Treaty of Guadalupe Hidalgo was negotiated, awarding California and New Mexico to the United States. This was opposed by some Northern Whigs who saw it as an opportunity to expand slavery. The Wilmot Proviso was proposed in 1846 to prohibit slavery in the newly acquired territory; however, it did not pass the Senate, which increased feelings of sectionalism. The Mexican War was opposed in its entirety, as the North saw it as a Southern plan to expand slavery. Another political misstep was the Ostend Manifesto, when President Franklin Pierce sent three diplomats to secretly buy Spain. Northerners saw the expeditions...