In 1825 President John Quincy Adams appointed Joel Poinsett as the first U.S. minister to Mexico. His first assignment was to persuade the Mexican government to sell the U.S. the province of Texas, thus continuing the rapid expansion of the American democracy. The United States continued to pursue Texas with little success for the next 20 years. It was not until December 1845 when the U.S. finally annexed Texas by a joint resolution (and thus simple majority) . Immediately following the Texas acquisition, and with U.S.-Mexico relations swiftly deteriorating, the U.S. wanted the Mexican province of California, mainly for her harbours San Frasisco and San Diego. The American policy towards Mexico which ensued in the following years was governed almost exclusively by President James Polk's personal opinions and actions, as well as Nicholas Trist's defiant behavior; a manifestation of the state-centric theory in which key individual decision makers govern policy. In addition, Polk's policies were secondarily influenced by the consideration of relative power, American mass ideology, and Public opinion.

In 1845 President Polk began, cofidentially from the public, considering the annexation of California. Polk's initial desire was to simply purchase California, attempting to maintain peace. He soon learned this would be impossible. When Polk ordered General Taylor to cross the Nueces River and eventually to fortify on the Rio Grande, he fully understood the possilble consequences of these actions. In fact, by deploying Taylor and his troops, Polk putting a slow squeeze on the Mexicans which would leave them with no other option than to strike back. Polk waited for the initial attack to be made by the Mexicans and then struck back. Polk claimed that American blood had been spilled on American soil, thus garnering enough public and congressional support to declare war on Mexico safe from domestic unrest. Norman Graebner states that, " Polk was too astute a politician to favor any cause until public opinion had crystallized "1 Although the war decleration contained no reference to the territorial conquest, Polk's persaonal diary conveys his clandestine intentions of acquiring the much coveted California as well as New Mexico. The intentions of the President to occupy Mexico undoubtedly took into consideration public opinion, but the most prominent reason for the decleration of war was Polk's belief that california was a strong economic and militarily strategic addition to the U.S. Secretary of Navy George Bancraft noted that the acquisition of California was among Polk's top four priorities from the outset of his administration, however, Polk had kept this under wraps. Glenn Price also points out that, "...the Mexican War was a result of President Polk utilizing Texas as a means to achieve annexation of California. "2 It is quite evident that Polk may have been following his own personal agenda in regards to acquiring Mexican territories, and beginning a war to do so.

Unquestionably the most important domestic issue in the years prior to the American Civil War was slavery. John C. Calhoun recognized that, ...if the treaty ending the conflict was silent on the subject of slavery in the ceded territory, the North will oppose it, and if it should prohibit slavery the South would, and in either event there would be a constitutional majority.