The great Irish potato famine forces huge waves of starving immigrants to the United States, sparking anti-Catholic, nativist backlashes.

General Zachary Taylor receives orders from Polk to move his troops from Fort Jesup in Louisiana to a position “on or near the Rio Grande” in Texas to discourage a Mexican invasion.

New York Jacksonian Democrat, John L. O'Sullivan, accuses opponents of Texas annexation of “limiting our greatness and checking the fulfillment of our manifest destiny to overspread the continent allotted by Providence for the free development of our yearly multiplying millions.” The term, “Manifest Destiny,” quickly becomes the label used to encapsulate the belief among Americans in their country's God-given right to expand its territory and institutions.

Under the direction of Secretary of the Navy George Bancroft, the Naval Academy opens at Annapolis, Maryland.

Texas is admitted as a slave state, making it the twenty-eighth state in the Union.

The Mormon migration to Utah, led by Brigham Young, begins.

Congress declares war on Mexico after American troops, under General Zachary Taylor, clash with Mexican troops on the north bank of the Rio Grande.

United States Declares War on Mexico

On May 13, 1846, President James K. Polk signed a declaration of war against Mexico. Polk had submitted his war message to Congress on May 11 after General Zachary Taylor and his troops had clashed with Mexican forces on the northern bank of the Rio Grande, and Congress quickly approved the declaration of war against Mexico. After the President signed the declaration, he and his cabinet decided to conduct a three-pronged war: General Taylor would secure northern Mexico, an army under Stephen Kearny would capture New Mexico and California, and a third force under the command of Winfield Scott would capture Mexico City.

Kearny headed west and found New Mexico abandoned by Mexican forces. He then moved to California, capturing Los Angeles in January 1847. Taylor remained active in northern Mexico, winning several battles and capturing Monterrey, an important Mexican trade-center. Polk then ordered a large portion of Taylor's troops to Vera Cruz to bolster Scott's force for the assault on Mexico City. Hearing of Taylor's reduced forces, Mexican General Santa Anna decided to attack but Taylor and his outnumbered troops repulsed the Mexican forces at the Battle of Buena Vista. Santa Anna retreated south and was again defeated when Scott captured Mexico City in September 1847.

With a strong advantage in the field, Polk's diplomat Nicholas Trist attempted to arrange terms with Mexico. After several false starts, Trist on February 2, 1848, arranged the Treaty of Guadalupe Hidalgo, which ended the war. Under the terms of the treaty, Mexico agreed to recognize the Rio Grande as the Texas border and ceded California and New Mexico to the United States. The United States agreed to pay Mexico $15 million for the territories and assume $3 million in outstanding American claims against the Mexican government. The Senate approved the treaty on March 10, 1848.

The Mexican War was both controversial and compelling at home. Many Whigs, including a young congressman named Abraham Lincoln, objected to the war, but these dissenting voices were mostly lost underneath an outburst of nationalism that only grew more vociferous as the American victories mounted. Propelling this nationalist upsurge was the penny press, which sent war correspondents to the field and made the war the most reported in American history to that date. The press coverage made General Zachary Taylor a hero who captured the public's imagination and helped to propel him to the presidency in 1848.

In the Bear Flag Revolt, approximately thirty American settlers (anticipating the Mexican War) take over a small Mexican garrison in Sonoma, California, and declare California a free and independent republic.

The Oregon Treaty establishes the 49th parallel as the border between British and American claims to the Oregon Territory, granting the United States clear title to present-day Idaho, Washington, Oregon, and Montana, while granting to Britain territory above the 49th parallel and full control over Vancouver Island.

Senate Approves Oregon Treaty

On June 15, 1846, the United States Senate approved the Oregon Treaty, which established the 49th parallel as the border between British and American claims to the Oregon territory. The treaty granted the United States clear title to present-day Idaho, Washington, Oregon, and Montana, and granted Britain the territory above the 49th parallel and full control over Vancouver Island.

James K. Polk gained the presidency in 1844 in part on the Democratic Party's expansionist pledge to seize all of the Oregon territory for the United States. America had jointly occupied Oregon with Britain since 1818, when the two nations began negotiating over a final boundary in the territory. Both sides had remained unwilling to agree to a dividing line which did not include for them the valuable harbor of Vancouver Island: the United States desired the more northerly 49th parallel, while Britain wanted the Columbia River to be the border (which was far south of the 49th parallel).

Two issues complicated matters in the 1830s and 1840s. First, American settlers poured into the region. Second, Americans who espoused the doctrine of Manifest Destiny called for the United States to take all of the Oregon territory, which ran to the 54º 40' line. Politicians felt compelled to respond to a now pressing political issue. President Tyler called for the annexation of Oregon to its northern limit, and Polk expressed a similar hard-line stance in his Inaugural Address; Polk's words inflamed American passions and upset the British.

In July 1845, however, Polk proposed a compromise to the British, offering to establish the boundary at the 49th parallel while granting Britain full control of Vancouver Island. With war in Mexico appearing ever more likely, the President wanted to avoid a simultaneous war with Britain. The British rejected the initial offer, and Polk responded by intimating that he was willing to go to war over the issue. London, however, reconsidered the proposition a year later, sending Polk a treaty on the terms he had proposed. When it arrived in Washington, Polk forwarded the treaty to the Senate unsigned, refusing to commit himself to it politically until the Senate approved it.

The Oregon Treaty was a great success, finally granting the United States clear title to vast tracts of land in the Northwest. Moreover, it allowed both Britain and the United States access to the Pacific Ocean through the channel south of Vancouver Island and avoided a possible war. Polk took criticism from some for abandoning the “All-Oregon” position, but for most it was a welcome settlement of the affair. Polk had handled the matter with skill, and the treaty allowed him to shift his full attention to the ongoing war with Mexico.

Congress passes the Tariff of 1846, a key part of President's Polk's domestic agenda. Known as the “Walker Tariff,” after Polk's secretary of the Treasury, Robert J. Walker, the Tariff of 1846 lowers rates toward revenue-only levels, although a few items remain protected.

President Polk vetoes a river and harbors bill which would have provided for federally funded internal improvements. Like Andrew Jackson in his veto of the Maysville Road bill, Polk argues that the bill unfairly favors particular areas, including ports which have no foreign trade. Therefore, says Polk, the bill is unconstitutional.

President Polk signs into law the Independent Treasury, which he calls a “Constitutional Treasury,” an integral part of his domestic agenda.

Congress establishes the Smithsonian Institution.

The Whigs regain a majority of seats in the House of Representatives.

Iowa is admitted as a free state, making it the twenty-ninth state in the Union.

General Zachary Taylor defeats the Mexicans under General Santa Anna at the Battle of Buena Vista. Taylor's victory cements his growing acclaim as a national hero and helps propel him to the 1848 Whig nomination for President.

The first Mormon settlers arrive in the Salt Lake Valley.

General Winfield Scott takes Mexico City, adding pressure to the Mexican government to submit to the demands of President Polk and sign a treaty of peace.

James Marshall discovers gold near Sacramento, California. The discovery begins the massive migrations of the California gold rush, allowing the territory to become a state and setting off fierce debates over whether to admit California as a free or slave state.

The Treaty of Guadalupe Hidalgo ends the Mexican War and grants the United States vast territories, including all or large parts of present-day California, Arizona, New Mexico, Colorado, Nevada, Utah, and Wyoming, and confirms the incorporation of Texas as part of the United States. The acquisition of these lands aggravates growing sectionalism in the country over the future of slavery in the Union.

Treaty of Guadalupe Hidalgo Signed

On February 2, 1848, Nicholas Trist-a special emissary dispatched to Mexico by President James K. Polk-signed the Treaty of Guadalupe Hidalgo with the Mexican government. The treaty ended the Mexican War, which began in 1846. Under the terms of the treaty, Mexico agreed to recognize the Rio Grande as the Texas border and ceded California and New Mexico to the United States. The United States agreed to pay Mexico $15 million for the territories and assume $3 million in outstanding American claims against the Mexican government.

Following General Winfield Scott's victories at Contreras and Churubusco in August 1847, Trist began negotiating a peace settlement to end the Mexican War. The next month, General Scott captured Mexico City. Still the Mexican government refused President Polk's terms for a peace treaty.

The slow progress of talks, as well as Trist's budding friendship with the Whig General Scott, frustrated Polk, and in October he ordered Trist to abandon negotiations and return to Washington, D.C. When Trist received the order in mid-November, he decided that Polk did not understand the situation in Mexico and ignored the recall notice. He reasoned that he could secure a treaty and the U.S. government would still be at liberty to reject it.

Polk was livid when Trist ignored his order to return to Washington, but he was nevertheless pleased when Trist signed the treaty with Mexico in February. At the suggestion of his cabinet, President Polk forwarded it to the Whig-controlled Senate. The treaty passed 38 to 14 on March 10, 1848, despite efforts from some Democrats (who wanted more Mexican territory) and some Whigs (who stood firmly against any acquisition of Mexican territory) to kill it. The treaty drastically enlarged the United States, granting the country more than 500,000 square miles of new western territory and valuable ports in California. The Treaty of Guadalupe Hidalgo was one of President James Polk's greatest accomplishments.

Wisconsin is admitted as a free state, making it the thirtieth state in the Union.

Elizabeth Cady Stanton and Lucretia Mott hold a women's rights convention in Seneca Falls, New York. Among other demands, the convention’s “Declaration of Sentiments” calls for women's suffrage.

Democrats regain a majority of seats in the House of Representatives.

Zachary Taylor is elected the twelfth President of the United States.

The United States Department of Interior is created. It combines the General Land Office, the Bureau of Indian Affairs, the Pension Office, the Bureau of the Census, and the Patent Office into a single department.

Zachary Taylor is inaugurated as the twelfth President of the United States, and Polk begins a goodwill tour of the South that will eventually end in Tennessee.