Origins

The United States' armed conflict with Mexico largely emerged from Americans' eagerness to expand their nation westward to the Pacific Ocean. As American trappers and settlers poured across the Great Plains, many began to resent the fact that lands to the south and west of the Louisiana Purchase tract remained territories of Mexico, which had freed itself from Spanish colonial control in 1821. Americans' persistent attempts to settle these lands led to conflict with the Mexican government and, eventually, war.
The new Mexican Republic had welcomed Americans to settle in their northern territory of Texas in the 1820s, but after a decade it became plain that the Americans resented Mexican rule. In 1835 the American settlers revolted against Mexico and, in the following year, established their own Republic of Texas. Many Americans immediately began to demand that their nation make Texas a part of the United States. The Mexican government warned that this would mean war.
In 1844 American elected James K. Polk as the nation's new president. Polk had campaigned on the issue of national expansion, calling for the annexation of Texas, Mexican California, and the Oregon Territory that the United States and Great Britain had occupied jointly since 1818. Just before leaving office in early 1845 President John Tyler, a Virginian seeking to provide a new area into which slavery might expand, secured a joint resolution from Congress annexing Texas to the United States. Mexico responded by breaking off diplomatic relations.
Upon taking office President Polk immediately turned to the acquisition of Mexico's northern territories. He first instructed his minister to Mexico to negotiate for the purchase of the territories, but this proposal sparked a wave of indignation and nationalist fervor in Mexico, and the minister left Mexico after only a few months.
Angry that Mexico had rebuffed his offer, Polk sent U.S. troops under the command of General Zachary Taylor to the Rio Grande River in January of 1846. Mexican officials believed that the Texas-Mexico frontier stood one hundred miles to the north, at the Nueces River, and interpreted Polk's move as a deliberate provocation. Mexican troops quickly arrived at the Rio Grande as well, and skirmishes broke out between the two forces. Polk leaped to argued that "Mexico… has invaded our territory and shed American blood upon American soil." Congress quickly provided him with a declaration of war.
In 1845 an American editor wrote that the American annexation of Texas represented the "fulfillment of our manifest destiny to overspread the continent allotted by Providence for the free development of our yearly multiplying millions." By 1846 newspapers across the country had appropriated the phrase in their attempts to show that God intended the American nation to stretch from Atlantic to Pacific.