The Mexican war began in 1846 due to the antagonistic ways of James K. Polk. Many people thought the conflict could be avoided and Polk came off that way also, but he seemed to taunt Mexican troops into crossing the Rio Grande and attacking our military first and Whigs questioned this claim. The one main thing that caused this war is the fact that Polk favored quick expansion and wanted to start with the annexation of Texas and he wanted to later gain California. Another big problem was that everyone was fighting over the Texas/Mexican border (Nueces or Rio Grande). The Mexican war was not strongly favored by any American because many felt that this was just going to tip the balance of slave states and free states when some western states became slave states. The defeat of the Wilmot Proviso soon came which raised suspicions of a Slave Power. This increased tensions between the North and the South which also led to the formation of the Free-Soil Party, favoring the Northerners take on slavery. The idea of Popular Sovereignty soon came into play which allowed territories to decide whether slavery should be allowed within their borders. This further displayed the increase in sectionalism.
With the new territories gained from the Mexican cession, Slavery was a big question. The people of the U.S fought to spread slavery, to stop slavery, and to slow the expansion of slavery, which caused sectionalism. Sectionalism and opposing views on slavery are essentially the causes of the war.
By Sloane