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 Federal Union; economic conflicts, due to industrialization in the North and agriculture in the South, over issues such as tariffs and internal improvements; and the bigger problem of slavery, which the South defended. For example, following the Mexican War, the Treaty of Guadalupe Hidalgo was negotiated, giving the United States California and New Mexico. This was opposed by some Northern Whigs, who saw it as an opportunity to expand slavery. The Wilmot Reservation was proposed in 1846 to prohibit slavery in the newly acquired territory; however, it did not pass the Senate, increasing the sense of sectionalism. The Mexican War was completely opposed, as the North saw it as a plan by the South to expand slavery. Another political misstep was the Ostend Manifesto, when President Franklin Pierce sent three diplomats to buy Spain in secret. Northerners saw the expeditions in...