The next great continental expansion of the United States comprised the entire western third of the lower forty-eight states, from the west boundary of the Louisiana Purchase to the Pacific Ocean. It was accomplished in only five years, from 1843 to 1848, but it was foreshadowed and prepared for by several migrations of American people, the spontaneous formation of governments on the American model, and the spread of an infectious propaganda resting on the brash assumption that American absorption of the desired territory was inevitable—that the Almighty had destined the territory for His "chosen people" as a kind of divine favoritism. This idea, like so many others in American colonialism, was inherited from the mother country. In the sixteenth century, the writings of Richard Hakluyt and others set forth the idea that, like the earlier Romans, Englishmen were preordained to take over, colonize, and develop the New World; no other people were capable of such a gigantic task. Destiny could be used just as easily to support American independence as to rationalize British imperial expansion, and revolutionary thought in the colonies was filled with echoes of the "city on a hill" and the "American Israel" blessed by Providence. Thomas Paine's statement in Common Sense that the Americans were fighting "the cause of all mankind" was only the most famous of many such declarations.
This potent propaganda, together with migration, American-style government, and an overall sense of an inevitability went to work in Texas during the 1820s. Texas had been a province on the north border of New Spain. When Mexico took over in 1821, its leaders thought to replicate the prosperous growth of the United States by encouraging the migration of American settlers as models into sparsely populated Texas, despite the warnings of a few prescient critics that this would amount to "settling the Goths at the gates of Rome." Americans, mostly Protestants speaking only English, gladly settled on the fertile cotton bottoms of east Texas and brought their slaves with them. (Mexico was officially Roman Catholic and Spanish speaking, and its constitution declared all men equal.) The new American population established commercial ties with New Orleans, since the nearest Mexican centers lay far to the south, and the people of central Mexico had little interest in Texan affairs, being much involved in their own political struggles. Frictions led to quarrels, and despite the efforts of a few Texan leaders to patch up disputes, the province revolted for independence in 1836. The Mexican president, Antonio López de Santa Anna led an army north to crush the rebels into submission, only to blunder into defeat at the Battle of San Jacinto in east Texas. Santa Anna was captured and forced to sign a treaty recognizing Texan independence.
Evidence of overt U.S. participation in the Texan revolution is slim. President Andrew Jackson's public behavior was correct, but the Texan general, Sam Houston, was his friend, and he followed the military action with keen interest. The American people, especially in the lower Mississippi Valley, were very sympathetic and contributed money and supplies freely; frontiersmen flocked into the Texan forces. An American general, Edmund P. Gaines, led a few troops fifty miles across the border but contributed nothing to Texan success. Not surprisingly, some Mexican nationalists have always believed that U.S. intervention gave the Texans their victory, and a small plot thesis has grown up among American historians, but their evidence is shallow.
Most of the victorious Texans hoped for prompt incorporation into the American Union, but this proved impossible because the United States was then implacably divided over the question of adding slave territory. In Europe, Britain and France opposed letting the United States strengthen itself through annexation. The British wanted an assured supply of cotton for their mills, and some inconsistently supported Texan abolitionism, as well. These contrary forces produced nine years (1836–1845) of frustrating diplomacy, while the ramshackle republic of Texas maintained a precarious existence, its government housed in log cabins on the western frontier, and a weak American president, John Tyler, tried to put together an annexation coalition to ensure his reelection.
In 1844, Tyler managed to sign an annexation treaty with Texas, only to see it defeated in the American Senate, partly because it came to a vote in the midst of a hard-fought presidential election campaign. The victor was an expansionist Democrat, James K. Polk. He favored annexing Texas, but Tyler tried to anticipate him during the three months he had left in office. In his hurry, he used an untried method, a joint resolution in both houses of Congress. At the same time, a diehard British agent in Mexico made a last-minute effort to secure that country's recognition of Texan independence. His failure and the Texan acceptance of terms offered by the joint resolution brought the annexation question to a dramatic conclusion.
On the Pacific coast the process of annexing California and Oregon was going on while that in Texas was drawing to a close. After Mexico won its independence it neglected its distant border provinces, and in the early 1830s American merchants established trading posts along the coast at Monterey, California, and a few other small ports, which became centers of American influence. At Monterey, Thomas O. Larkin, the most important merchant, became a U.S. consul and an important source of information about conditions in California. By the 1840s these merchants were joined by American explorers, hunters, and trappers crossing the mountains. They soon developed another center of American influence at a backwoods fort near modern Sacramento built by John A. Sutter, a German Swiss, who obtained a Mexican land grant in 1840 and founded a polyglot settlement. The Hudson's Bay Company, moving southward from western Canada, formed the third corner of the triangle competing for influence in California. The Mexican government maintained loose control over these disparate elements with little or no supervision from Mexico City. Americans on the East Coast exaggerated the European, especially the British, threat to California, and even Anglophile Daniel Webster tried to include in his negotiations with Ashburton some provision for the transfer of the San Francisco Bay area to no avail. The degree of American nervousness about California was shown by Thomas Catesby Jones, commanding the U.S. Pacific Squadron. In 1842, hearing a false rumor of an Anglo-American war, he took it upon himself to seize the port of Monterey, only to have to back out in an atmosphere of general embarrassment.
To the north of California, Americans were also much interested in the vaguely defined Oregon Territory, which the United States had occupied jointly with Britain since the northern boundary settlement at 49 degrees (1818). The British, who had explored and mapped the area as far as the Alaska border, had a better claim to it, but an American ship captain had discovered the mouth of the Columbia River, and the Lewis and Clark expedition (1804–1806) had explored the Columbia and Snake River valleys. Five years later, John Jacob Astor established a fur trading post and fort at Astoria, near the mouth of the Columbia, and in the Florida treaty of 1819 the United States acquired the Spanish claim south of the river. After that time, the Oregon question largely revolved around the triangle of land north of the Columbia River (modern Washington State), especially the coasts of Puget Sound, with their excellent harbors.
For the time being the British established monopoly control over the Columbia Valley through their fur trade and agriculture, and they even furnished food for the Russian settlements in Alaska. Meanwhile, the Americans, led by westerners in the House of Representatives, tried to alert their countrymen to the danger that Britain might forestall them in Oregon. In 1827 the two nations reviewed the whole question. The United States offered to extend the 49-degree line (established in 1818) and cede all of Vancouver Island to Britain (the eventual settlement); Britain offered only several ports on the Olympic peninsula, which its navy could easily take over in wartime. The joint occupation continued, and the Hudson's Bay Company remained the de facto ruling power in Oregon. To the disappointment of both governments, Webster and Ashburton decided not to included Oregon in their negotiations.
At this point the tide began to turn. A new British treaty with China raised trade possibilities in Asia, and British colonies in New Zealand and elsewhere began to compete for settlers with Puget Sound; also, the Hudson's Bay Company's interest in the lower Columbia valley began to cool. At the same time American concern over Oregon increased, and a new burst of speeches and memorials forced the issue on the public. Missionaries were sent out to work among the Oregon Indians, and their enthusiastic reports on the beauties of the country stimulated the migration of families and sometimes whole communities to Oregon. A trail of sorts had existed as early as the War of 1812, and by the 1830s it was an institution—up the Platte River, across the prairies and mountains, through South Pass, down the Snake River, and finally to the Columbia. (At Fort Hall an alternate route led into California.) Many travelers went as far as a tributary of the Columbia, the Willamette, whose banks resembled parts of the Middle West so much that it became a common goal. The climax of the migration came in 1843 as returning prosperity made possible improvements in equipment and supplies. To many observers it seemed that time was working for the Americans, and a group of Southern Democrats led by John C. Calhoun spoke out for "a wise and masterly inactivity." At that point Calhoun became Tyler's secretary of state. Since Britain had about concluded that the 49-degree line was the best it could obtain at that time, preliminary negotiations began between Calhoun and the British minister, but these became hung up on details, and soon the presidential election of 1844 intervened. In the campaign, western activists kept alive their demand for all of Oregon up to the boundary of Russian Alaska—a grant of land altogether unjustified by circumstances and likely to cause war with Britain.
The new president, Polk, brought about a wholly unnecessary crisis with Britain as he maneuvered to keep together the two factions of his party, the activists (western) and the pacifists (southern, led by Calhoun). First he took advantage of a British misstep to close off Tyler's negotiations, in which he thought the American position was too weak. Then he referred the whole question to Congress, so the western fulminations could alarm the British. It was at this point, and not earlier, that the slogan "Fifty-four forty or fight" spread across the country. After a heated debate, Congress voted to send Britain a mild, unthreatening notice abrogating the joint occupation. The British took the initiative, as Polk had intended, and the diplomats quickly settled on the forty-ninth parallel, awarding all of Vancouver Island to Britain. The British gave up the right to navigate the lower Columbia, avoiding possible commercial friction in the future. Thus, Polk carried off his policy through rigid party discipline but he risked a costly war for purely partisan gains.
American migration into California during the early 1840s, though smaller than that into Oregon, continued to tip California toward the United States, thanks especially to the publicity given to the Sacramento–San Joaquin area by the well-known explorer John Charles Frémont, whose report on his two expeditions was a bestseller. In September 1844 a group of native Californians with some Americans overturned the local Mexican governor and set up their own virtually autonomous government. When Polk took office in March 1848, he intended to negotiate for California, and the War Department sent Frémont back into the province. The British also dispatched a warship to visit the north Pacific and be on hand in case of a local revolt. For its part, the Mexican government gathered several hundred soldiers as reinforcements for California, but on their way to the West Coast they encountered a local uprising and joined it, bag and baggage.
As the Texas question wound down in 1845, Polk set his Mexican policy in motion. It was both aggressive and defensive at the same time—to mount a strong front, assume the benefit of every doubt, and challenge every assumption of his opponent, while avoiding any overt act that might commit him beyond recall—a policy of bluff, indeed, but bluff with an avenue of retreat. Since Mexico had broken relations with the United States over the annexation of Texas, Polk sent an envoy to start negotiations, although by usual practice Mexico should have acted first. The envoy, John Slidell, was chosen largely for his fluency in Spanish. Polk instructed him to offer $5 million for the cession of Texas to the Rio Grande and New Mexico and $25 million for California, including San Francisco and Monterey. The president indicated later that Slidell might bargain beyond the figure for California but he made no mention of an explanation or indemnity for the already completed annexation of Texas. Even before Polk drew up these instructions for Slidell, he took care that his envoy should negotiate from a position of strength by stationing troops under General Zachary Taylor at Corpus Christi in southeast Texas.
In early October news arrived from Polk's agent in California, the merchant Thomas O. Larkin, that forced Polk to hurry his plans. Larkin described British and Mexican activities that seemed to point to a British seizure of the province. It is now known that Larkin's information, five months old, was based on rumor and entirely false, but it must have deeply shocked Polk and his cabinet. He sent off new instructions to Taylor, Larkin, and the commander of the Pacific Squadron, John D. Sloat. Taylor was to advance to the Rio Grande (the farthest line claimed by Texas). Sloat was to conciliate the Californians, keep his ships ready for further orders, and, if war broke out, seize the principal California ports; he was also reinforced. Larkin became confidential agent to report on developments and defeat any foreign efforts to control California. Separate orders were sent to Frémont in California. No one has discovered what these orders were or in particular how much freedom they gave to a notoriously flighty man of action.
Having set out his lines, Polk now had to wait to see their effect, while Congress plunged into debate over Oregon. Slidell arrived in Mexico at the beginning of December to find that the president, who favored negotiating but was only clinging to power, would not receive him. A new regime, more nationalistic and stubborn, soon took power, and Slidell resigned himself to waiting outside Mexico City and reporting on the situation. Meanwhile, General Taylor and his troops were marching to the Rio Grande, which they reached in March. They built a temporary fort across the river from the Mexican town of Matamoros (modern Brownsville, Texas). The Mexicans responded by sending a reconnaissance party across the river. On 25 April this force clashed with an American scouting party, took some prisoners, and shed the first blood of the war. At the same time, Polk recalled Slidell and he and news of hostilities on the Rio Grande arrived in Washington at the same time. On a Sunday morning, Polk, glad to have some justification for acting, called his cabinet together and drew up a war message. In this he branded the Mexicans as the aggressors and the conflict as a defensive one, "war by the act of Mexico." Polk's enemies in Congress, the Whigs and Calhoun, would not accept this, but Polk's Democrats introduced appropriation bills and other measures to support the troops, and presently patriotism blurred the distinctions, and a formal declaration of defensive war crept through. (Mexico too declared a defensive war.)
When the American cabinet discussed the goals of the war, the cautious James Buchanan (secretary of state) proposed a circular letter disavowing any territorial ambitions beyond the Rio Grande boundary for fear of a British or French intervention. Polk firmly vetoed such a statement. By the end of May he had decided on the annexation of all Upper California, New Mexico, and perhaps other parts of northern Mexico. As soon as Congress passed the war bill, Polk ordered a blockade of the California coast and sent General Stephen W. Kearny, then in Missouri, to lead troops westward—eventually to occupy California. Taylor's initial victories over Mexico (Palo Alto and Resaca de la Palma), as well as the settlement of the Oregon question, reduced the danger of British intervention in California. During June, before the news of war reached the west coast, Frémont touched off a revolt against the local authorities and captured the town of Sonoma—whether on instructions from Washington, no one knows for certain. Commodore Sloat after some hesitation proclaimed American rule. (Actual conquest, however, would await the arrival of General Kearny and his regular troops.) Events in the principal theater of war had less connection with continental expansion. Taylor pressed southward through Matamoros, Camargo, Monterrey, and Saltillo, but a fierce battle at Buena Vista in February 1847 convinced Polk that the army had reached a dead end. He created a second army, commanded by Winfield Scott, which landed at and captured Veracruz, then pushed into the interior of the country toward Mexico City. Reaching the central valley of Mexico, Scott finally occupied the capital in September 1847.
Polk had not originally intended to go so far. He expected a short war, having no idea of the Mexican sense of honor or stubbornness. So the war became a series of advances followed by pauses in which action was stopped to see if the Mexicans would yield. During one of these pauses, after Scott had occupied Veracruz, Polk appointed Nicholas Trist to join Scott as a commissioner and be ready to act as soon as the Mexican showed any interest in negotiating. (Trist was a minor official in the State Department. Polk did not care to send the elderly Buchanan so far away.) After the American capture of Mexico City, the Mexican government, by now controlled by moderates, retreated to an interior town. An important force working for peace was the British legation, because after the fall of Veracruz, the British government had resigned itself to an American victory and was now anxious to end the war as quickly and cheaply as possible. Trist, Scott, and Mexican representatives with British advisers discussed terms in Mexico City, and a man on horseback carried these to the Mexican government a hundred miles away for further discussion. Polk played no active role in the negotiation of terms, and, indeed, was so disgusted with the proceedings that at one point he ordered Trist to stop negotiating and come home. Fortunately, Trist disobeyed; for this, the vindictive president stopped his pay (Congress made an appropriation twenty-two years later.)
The treaty so torturously produced kept surprisingly close to Polk's early idea of the war's goals. It ceded to the United States all of Upper California and New Mexico and drew the U.S.–Mexico boundary at the Rio Grande and Gila Rivers. In return the United States was to pay $15 million and assume $3.25 million of American claims against Mexico. The cessions did not include Lower California or the Isthmus of Tehuantepec, which had been mentioned as a transit route for a railroad or canal, but the United States had not occupied these territories, and annexing them at that time would have presented serious problems. Considering the sectional rivalries the country faced, it had taken perhaps too much as it was. During the last months of the war, a movement grew in the United States to annex all of Mexico as the only way to reimburse the nation for the expense of the war. In addition, among the divided Mexican parties one left-wing group favored an American protectorate as the only way of pushing through changes such as clerical reform, which Mexico badly needed but could not manage by itself.
When Polk saw the treaty signed by his disobedient agent, he was furiously disgusted, but he swallowed his rage and transmitted the document to the Senate with minor changes. A coalition of extremists who wanted to annex more of Mexico and who wanted a peace without annexations threatened to defeat the treaty and throw the negotiations open again. Proceedings were delayed by the sudden death of John Quincy Adams in the House chamber and this may have provided time for common sense to prevail. Eventually, moderates gathered themselves together and approved the Treaty of Guadalupe Hidalgo with Polk's amendments by a vote of 38 to 14.