In 1848 Mexico sold almost half of her territory to the United States through the Treaty of Guadalupe Hidalgo. The US gained a vast tract of land, and more importantly, a lengthy stretch of the Pacific Coast. California was truly the prize in the United States' war against Mexico, and its acquisition proved to be the fulfillment of American expansionistic goals, attainment of which in the 1840s was motivated by the evolving, almost moral doctrine of Manifest Destiny, as well as geopolitical realities.
The notion of Atlantic to Pacific Manifest Destiny was not a new one in the 1840s. Indeed, most of the charters of the original British colonies were "sea to sea," meaning that while north and south boundary lines were established, the colony would theoretically extend all the way to the Pacific ocean. The "West" has traditionally been an escape valve for population, and the promise of cheap or free western land has always been part of the American appeal. The nation's population was already inexorably pushing westwards before the movement gained new momentum in the 1840s, and the Union admitted 15 new western states between 1783 and 1845. At its heart, Manifest Destiny was simply the natural result of population pressures, which pushed Americans farther and farther west seeking land and prosperity.
Manifest Destiny metamorphosed into a moral doctrine as American journalists and writers began to celebrate the westward movement during the 1840s. The term "Manifest Destiny" itself was coined by Tim O'Sullivan in 1845, and around that same time the New York Tribune's Hoarce Greeley popularized the phrase "Go West Young Man." Politicians and newspapers proclaimed it the duty of Americans to spread their unique way of government across the continent, and soon the notion was being alluded to by some as a God-given mission, as Americans, as if quoting Genesis (as propounded by John Quincy Adams in an expansionistic speech), saw the west as a place to "be fruitful and multiply, and replenish the earth and subdue it." 1
Manifest Destiny was driven by individual wanderers rather than any higher plan of conquest or colonization. Thus American settlers established themselves on the lands of the British (Oregon), Mexicans (California) and Native Americans (everywhere), then left the government to sort out the military and diplomatic repercussions. Such ad hoc expansion made plausible by the fact that there was really no one to stand in the way. With the dissolution of the Tecumseh Confederacy in 1811, there was no well-organized Native American opposition against white encroachment, and though the Indians fought tenaciously, they were overwhelmed by waves of land hungry settlers. The Mexican population of California in the 1830s and 40s was a miniscule 7000. The British and Russians maintained trading outposts on the West Coast, and the high British population in Northern Oregon caused it to be partitioned with the United States by treaty in 1846, creating the Province of British Colombia. However, most the North American west was effectively open land (although others held claim to it), ripe for American settlement.
Expansionism found a powerful proponent in the person of President James Polk. Polk was the first dark horse candidate in United States history and also the last strong president prior to the Civil War. A former lieutenant of Andrew Jackson, he was closely linked with Jackson politically, and sported the moniker "Little Hickory" after his political mentor. He came with an ambitious agenda (all of which he accomplished), which included lowering the tariff, shoring up the national money supply, and resolving the tricky issue of the Oregon border with Great Britain, all of which he had promised in his campaign. However, he kept his most important goal private, confided only to a handful of cabinet members: the acquisition of California.
Polk was not particularly motivated by the intangible, frequently religious bluster of Manifest Destiny, but rather by highly palpable geopolitical aims. Polk was keenly interested in obtaining Pacific ports for the American naval and merchant fleets. The US acquired commerce rights in China in 1844, and thus newly opened Chinese ports beckoned to American merchants, who would benefit tremendously from possession of West Coast berths, which would also serve as commerce protecting naval bases.2 Thus, when settling the Oregon dispute with the British in 1846, Polk was unwilling to go to war for the 54th Parallel (despite the hard liner rallying cry of "54 or Fight!"), as there were no port or harbors of any serious consequence in the northern part of the Oregon territory. However, he stuck staunchly by the American demand for a border at the 49th parallel, giving American ships access to the Puget Sound.3California offered even more promising harbors: San Francisco Bay was the finest and largest bay of its type on the Pacific ocean, and San Diego Bay was likewise an large and highly sheltered bay.
American prestige was also at stake. An empty West Coast was vulnerable to colonization by a third party, particularly Britain. With the issuance of the Monroe Doctrine in 1824, the US bound itself to prevent any further colonization on the Western Hemisphere. The acquistion of the West Coast by Britain or any other nation would be a slap in the face to both the Monroe Doctrine and the standing of the United States, particularly because in the 1840s the Monroe Doctrine was still a bluff, as the US utterly lacked the relative military power to enforce it.4
In truth, there were no serious aspirations by other nations on the Western Hemisphere. Russian fur trading outposts, such as Fort Ross in Northern California, were technically violations of the Monroe Doctrine (which had been initially issued in response to Russian West Coast encrouchment), but the Russians were never present in any serious numbers. Indeed they eagerly liquidated their unprofitable North American enterprises, selling off Fort Ross to John Sutter in 1836. The British presence in Oregon was not in violation of the Monroe Doctrine (a treaty in 1810 had called for the joint occupation of the Oregon territory between the US and Britain), but was still troublesome to American policy makers; hence Polk''s urgency to settle the matter diplomatically.
The Americans managed to create a bogeyman out of the British, and US policy was frequently guided by the false and frequently paranoid notion that the British fleet was constantly ready to pounce upon California. After all, the British were already heavily invested in China with the conclusion of the First Opium War in 1842, and hence the Royal Navy was active in the Asiatic. California bases thus could ensure British control of the entire breadth of the Pacific, and forever dash American ambitions on the West Coast.
American fears were based primarily on nervous speculation coupled with rumors and misinterpretation of evidence, rather than actual British intent. The British government was in no mood for a war or confrontation with the United States, as shown by their willingness to compromise on the Maine boundary in 1837 and on the Oregon territory in 1846. However, the jumpy Americans imagined or misinterpreted a wide array of British plots against California. British adventurers and merchants published alarming books advocating colonization of California, despite the fact that these publications did not represent official British policy makers , who realized that any investment in California would represent pointless over-extension.5 When the Irish Priest Eugene MacNamara proposed to the Mexican Government that thousands of Irish refugees be allowed to settle in California, Americans, in a curiously contradictory way, decried the move as both a British and papal plot against the West Coast.6 The fact that both the British and the French maintained consuls in California, despite the lack of large scale commercial operations (the American consul, Thomas Larkin, was apparently justified due to the active hide and tallow trade with Yankee merchants) seemed to further confirm American suspicions of foreign intrigue.
When a British squadron left Peru in 1842 under sealed orders, the commander of the US Pacific Squadron, Commodore Thomas ap Catesby Jones, became convinced that the US and Mexico were with war and that the British were sailing to occupy California, taking advantage of the instability. Jones rushed his ships to Monterey, landed sailors and marines, and forcefully occupied the town for a day. Discovering, after careful review of official documents and correspondence, himself in error about the presence of a state of war, he apologized, fired a salute and then retired, but his hasty actions are demonstrative of the intense American anxiousness about losing California.
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