James K. Polk in the context of "Henry Clay"

The Mexican–American War, also known in the United States as the Mexican War and in Mexico as the United States intervention in Mexico, (April 25, 1846 – February 2, 1848) was an invasion of Mexico by the United States. It followed the 1845 American annexation of Texas, which Mexico still considered its territory because it refused to recognize the Treaties of Velasco, signed by President Antonio López de Santa Anna after he was captured by the Texian Army during the 1836 Texas Revolution. The Republic of Texas was de facto an independent country, but most of its Anglo-American citizens who had moved from the United States to Texas after 1822 wanted to be annexed by the United States.

Sectional politics over slavery in the United States had previously prevented annexation because Texas would have been admitted as a slave state, upsetting the balance of power between Northern free states and Southern slave states. In the 1844 United States presidential election, Democrat James K. Polk was elected on a platform of expanding U.S. territory to Oregon, California (also a Mexican territory), and Texas by any means, with the 1845 annexation of Texas furthering that goal. However, the boundary between Texas and Mexico was disputed, with the Republic of Texas and the U.S. asserting it to be the Rio Grande and Mexico claiming it to be the more-northern Nueces River. Polk sent a diplomatic mission to Mexico in an attempt to buy the disputed territory, together with California and everything in between for $25 million (equivalent to $798 million in 2024), an offer the Mexican government refused. Polk then sent a group of 80 soldiers across the disputed territory to the Rio Grande, ignoring Mexican demands to withdraw. Mexican forces interpreted this as an attack and repelled the U.S. forces on April 25, 1846, a move which Polk used to convince the Congress of the United States to declare war.

The Whig Party was a mid-19th century political party in the United States. Alongside the Democratic Party, it was one of two major parties from the late 1830s until the early 1850s and part of the Second Party System. As well as four Whig presidents (William Henry Harrison, John Tyler, Zachary Taylor, and Millard Fillmore), other prominent members included Henry Clay, Daniel Webster, Rufus Choate, William Seward, John J. Crittenden, and John Quincy Adams (whose presidency ended prior to the formation of the Whig Party). The Whig base of support was amongst entrepreneurs, professionals, Protestant Christians (particularly Evangelicals), the urban middle class, and nativists.

The party was hostile towards the ideology of "manifest destiny", territorial expansion into Texas and the Southwest, and the Mexican–American War. It disliked presidential power, as exhibited by Andrew Jackson and James K. Polk, and preferred congressional dominance in lawmaking. Members advocated modernization, meritocracy, the rule of law, protections against majority rule, and vigilance against executive tyranny. They favored an economic program known as the American System, which called for a protective tariff, federal subsidies for the construction of infrastructure, and support for a national bank. The party was active in both the Northern and Southern United States and did not take a firm stance on slavery, but Northern Whigs tended to be less supportive than their Democratic counterparts.

Presidential elections were held in the United States from November 1 to December 4, 1844. Democratic nominee James K. Polk narrowly defeated Whig Henry Clay in a close contest turning on the controversial issues of slavery and the annexation of the Republic of Texas. This is the only election in which both major party nominees served as Speaker of the House at one point, and the first in which neither candidate held elective office at the time.

President John Tyler's pursuit of Texas annexation divided both major parties. Annexation would geographically expand American slavery. It also risked war with Mexico while the United States engaged in sensitive possession and boundary negotiations with Great Britain, which controlled Canada, over Oregon. Texas annexation thus posed both domestic and foreign policy risks. Both major parties had wings in the North and the South, but the possibility of the expansion of slavery threatened a sectional split in each party. Expelled by the Whig Party after vetoing key Whig legislation and lacking a firm political base, Tyler hoped to use the annexation of Texas to win the presidency as an independent or at least to have decisive, pro-Texas influence over the election.

The Thornton Affair was a battle in 1846 between the military forces of the United States and Mexico 20 miles (32 km) west upriver from Zachary Taylor's camp along the Rio Grande. The much larger Mexican force defeated the Americans in the opening of hostilities, and was the primary justification for U.S. President James K. Polk's call to Congress to declare war. It is also known as the Thornton Skirmish, Thornton's Defeat, or Rancho Carricitos.

Presidential elections were held in the United States on November 7, 1848. Held in the aftermath of the Mexican–American War, General Zachary Taylor of the Whig Party defeated Senator Lewis Cass of the Democratic Party.

Despite Taylor's unclear political affiliations and beliefs, and the Whig opposition to the Mexican–American War, the 1848 Whig National Convention nominated the popular general over party stalwarts such as Henry Clay and Daniel Webster. For vice president, the Whigs nominated Millard Fillmore, a New York Whig known for his moderate views on slavery. Incumbent President James K. Polk, a Democrat, honored his promise not to seek re-election, leaving his party's nomination open. The 1848 Democratic National Convention nominated Senator Lewis Cass of Michigan after former President Martin Van Buren withdrew his bid for a second term over a platform dispute. Van Buren broke from his party to lead the ticket of the Free Soil Party, which opposed the extension of slavery into the territories.

The land which became the United States was inhabited by Native Americans for tens of thousands of years; their descendants include but may not be limited to 574 federally recognized tribes. The history of the present-day United States began in 1607 with the establishment of Jamestown in modern-day Virginia by settlers who arrived from the Kingdom of England, and the landing of the Mayflower by English pilgrims to Plymouth in 1620. In the late 15th century, European colonization began and largely decimated Indigenous societies through wars and epidemics. By the 1760s, the Thirteen Colonies, then part of British America and the Kingdom of Great Britain, were established. The Southern Colonies built an agricultural system on slave labor and enslaving millions from Africa. After the British victory over the Kingdom of France in the French and Indian Wars, Parliament imposed a series of taxes and issued the Intolerable Acts on the colonies in 1773, which were designed to end self-governance. Tensions between the colonies and British authorities subsequently intensified, leading to the Revolutionary War, which commenced with the Battles of Lexington and Concord on April 19, 1775. In June 1775, the Second Continental Congress established the Continental Army and unanimously selected George Washington as its commander-in-chief. The following year, on July 4, 1776, the Second Continental Congress unanimously declared its independence, issuing the Declaration of Independence. On September 3, 1783, in the Treaty of Paris, the British acknowledged the independence and sovereignty of the Thirteen Colonies, leading to the establishment of the United States. On September 17, 1787, the U.S. Constitution was signed by a majority of delegates, and was later ratified by the Thirteen Colonies, leading to the first modern U.S. government.

In the 1788-89 presidential election, Washington was elected the nation's first U.S. president. Along with his Treasury Secretary, Alexander Hamilton, Washington sought to create a relatively stronger central government than that favored by other founders, including Thomas Jefferson and James Madison. On March 4, 1789, the new nation debated, adopted, and ratified the U.S. Constitution, which is now the oldest and longest-standing written and codified national constitution in the world. In 1791, a Bill of Rights was added to guarantee inalienable rights. In 1803, Jefferson, then serving as the nation's third president, negotiated the Louisiana Purchase, which doubled the size of the country. Encouraged by available, inexpensive land, and the notion of manifest destiny, the country underwent westward expansion in a project of settler colonialism marked by a series of conflicts with the continent's indigenous inhabitants. The most notable advocate of manifest destiny was President James K. Polk, who annexed Texas in 1845, and declared war on Mexico the next year. An overwhelming U.S. victory in the Mexican-American War led to the Treaty of Guadalupe Hidalgo in 1848, where the U.S. acquired much of the American Southwest from Mexico. Whether or not slavery should be legal in the expanded territories was an issue of national contention, and led to increasing tensions over slavery.

The Mexican Cession (Spanish: Cesión mexicana) is the territory that Mexico ceded to the United States in the Treaty of Guadalupe Hidalgo in 1848 after the Mexican–American War. It comprises the states of California, Texas, New Mexico, Utah, Nevada, and Arizona, and parts of Colorado, Oklahoma, Kansas, and Wyoming in the present-day Western United States. Consisting of roughly 529,000 square miles (1,370,000 km), not including Texas, the Mexican Cession was the third-largest acquisition of territory in U.S. history, surpassed only by the 827,000-square-mile (2,140,000 km) Louisiana Purchase of 1803 and the later 586,000-square-mile (1,520,000 km) Alaska Purchase from Russia in 1867.

Most of the ceded territory had not been claimed by the Republic of Texas following its de facto independence in the 1836 revolution. Texas had only claimed areas east of the Rio Grande. After annexation into the U.S. and admission as a state in 1845, Texas's southern and western boundaries with the Mexican state of Santa Fe de Nuevo México had not been defined by the U.S. Most of the ceded area was part of the Mexican province of Alta California (Upper California) while the southeastern strip east of the Rio Grande had been part of the state of Santa Fe de Nuevo México. Mexico had controlled the ceded territories with considerable local autonomy. After the Mexican War of Independence finished in 1821, the area had seen several revolts but the Mexican government sent few troops from central Mexico to suppress them. In 1846, U.S. President James K. Polk sent soldiers to occupy the disputed territory between the Nueces River and the Rio Grande, starting the Mexican–American War. At the war's outbreak, the U.S. conducted a naval landing on Mexico's northeastern coast on the Gulf of Mexico, and advanced into Nuevo México.

The Democratic Party is one of the two major political parties of the United States political system and the oldest active political party in the country. Founded in 1828, the Democratic Party is the oldest active voter-based political party in the world. The party has changed significantly during its nearly two centuries of existence. Once known as the party of the "common man", the early Democratic Party stood for individual rights and state sovereignty, and opposed banks and high tariffs. In the first decades of its existence, from 1832 to the mid-1850s (known as the Second Party System), under Presidents Andrew Jackson, Martin Van Buren, and James K. Polk, the Democrats usually defeated the opposition Whig Party by narrow margins.

Before the American Civil War, the party generally supported slavery or insisted it be left to the states. After the war until the 1940s, the party opposed civil rights reforms in order to retain the support of Southern white voters. The Republican Party was organized in the mid-1850s from the ruins of the Whig Party and Free Soil Democrats. It was dominant in presidential politics from 1860 to 1928. The Democrats elected only two Presidents during this period: Grover Cleveland (in 1884 and 1892) and Woodrow Wilson (in 1912 and 1916). Over the same period, the Democrats proved more competitive with the Republicans in Congressional politics, enjoying House of Representatives majorities (as in the 65th Congress) in 15 of the 36 Congresses elected owing largely to their dominance of the Solid South, although only in five of these they formed the majority in the Senate. Furthermore, the Democratic Party was split between the Bourbon Democrats, representing Eastern business interests, and the agrarian party elements representing poor farmers in the South and West. After the Republican landslide in the 1894 House elections, the agrarian element, marching behind the slogan of free silver (i.e. in favor of inflation), captured the party. They nominated William Jennings Bryan in the 1896, 1900, and 1908 presidential elections, although he lost every time. Both Bryan and Wilson were leaders of the progressive movement in the United States (1890s–1920s) and opposed imperialistic expansion abroad while sponsoring liberal reforms at home despite supporting racism and discrimination against blacks in government offices and elsewhere.