The Mexican-American War began because there was a boundary dispute between the United States and Mexico over the border with Texas. The United States claimed the border with Mexico was at the Rio Grande River. The Mexicans claimed the border was at the Nueces River. Depending on each side’s claim, Texas would either be larger if we used the American border definition or smaller if we used the Mexican border definition.
The United States wanted to negotiate a settlement to this dispute with the Mexican government. We sent a representative, John Slidell, to meet with Mexican officials. We were prepared to offer Mexico $30 million for California and New Mexico if Mexico would accept the border at the Rio Grande River. However, the Mexican government wouldn’t meet with John Slidell. As a result, when the Mexicans attacked our troops that were in the disputed territory, we went to war against Mexico in what was known as the Mexican-American War.