In less than ten years, the United States succeeded in securing the western half of the current continental America. This sudden burst of expansion was not accidental; the turmoil of Mexico, domestic understanding of America’s destiny, and belligerent rhetoric of politicians acted as fuel for an expansionist engine that benefited America’s own interests. The 1840s presented a convergence of strategic advantages and moral values, putting the United States in a prime position to acquire territory.

Strategically, the United States found itself primed to expand. After the independence of Texas, a struggle began with Sam Houston’s call to drive “the oppressors… from our soil,” that may have been there “exclusively for the purpose of aiding the Texas rebellion.” Moreover, those rebels, including Houston and other men, were known to have “come from New Orleans and other points of the neighboring republic,” insinuating that the United States had passively supported the drive for independence. Post-independence, the United States had a more compelling argument for annexation. It was certainly in America’s interests to gain access to Texas; by outraged Mexicans themselves, “Texas… possesses all the elements requisite for prosperity in agriculture, industry, commerce, and navigation.” By aiding in independence, the United States knew it would be able to annex the fledgling country. Polk, in outlining his claim of Texas in 1845, slips in the true benefits of an American Texas, admitting the “vast resources of her fertile soil and genial climate would be speedily developed, while the safety of New Orleans and of our whole southwestern frontier against hostile aggression.” Tellingly, he points out “the interests of the whole union… would be promoted by it.” Polk damningly lists the interests being “perpetual peace,” “free intercourse,” and “unrestricted communication.” By his own admission, the cards the U.S. had been dealt (or selected itself) provided an excellent opportunity to bring Texas into the union. In the same speech, Polk makes a similar argument. This time, he argues the cards that were dealt were the “peaceful triumphs of the industry of our emigrants,” giving the United States reason to extend “our laws and the benefits of our republican institutions.” Polk did not settle for just these two territories. A year later, he pleaded Congress to declare war on all of Mexico after claiming that America’s neighbor had “invaded our territory and shed American blood on American soil.” This claim was dubious at best, as Mexico had not recognized that border in the first place. To Mexicans, “the strange idea…that the limits of Texas extended to the Rio Bravo del Norte” explained the misunderstanding. Regardless, the ultimate result of an ostensibly noble attempt to defend America was the additional acquisition of half of Mexico’s territory in 1848. Fortunate external circumstances, aggression at the disputed Texan border, and westward expansion of U.S. settlers allowed America a convincing path to further land and subsequent power.

The strategic acquisitions of land were not viewed as a wanton aggression; a moral framework lent credence to America’s actions. John L. O’Sullivan advocated this paradigm; in an 1839 journal article, he mixed American Exceptionalism with expansionism and Christianity into a concoction thereafter known as Manifest Destiny. According to O’Sullivan, with “the expansive future … in our arena,” who could “set limits to our onward march?” After all, “for this blessed mission to the nations of the world… has America been chosen.” O’Sullivan’s sugarcoating of strategically beneficial and attainable land made it irresistible; Manifest Destiny filtered its way into Polk’s own words. Before outlining why Texas was valuable to America’s strategic and economic interests, Polk pointedly says that enlarging America is tantamount to extending “the dominions of peace over additional territories and increasing millions.” To Polk, this extension functioned as a “reannexation of Texas.” In other words, it was taking back what America was originally destined to have. The tone of this rhetoric, in retrospect, appears patronizing. But O’Sullivan’s Democratic Review morally justifies what the U.S. desired, expansion. According to him, “the irresistible army of Anglo-Saxon emigration” was destined for all of the continent, including California. It is important to note that not every American bought into the expansionist destiny. In fact, Senator Daniel Webster warned in 1848 that expansion would be “likely to turn the Constitution under which we live into a deformed monster.” At the same time, he intelligently conceded that “whatever measure the Executive Government embraces and pushes, is quite likely to succeed.” Unfortunately for the Senator, his position was at odds with the executive in power.

It was a perfect storm. A moral justification to strategic imperialism unlocked the benefits both Mexicans and Americans knew came with the acquisition of additional territory. The external situation and alignment of realism with morality explains the massive land grab in the 1840s.