The United States annexes the Republic of Texas in 1845 during a period of expansionist Manifest destiny.

The 1846 Oregon Treaty with Britain leads to U.S. control of the present-day American Northwest.

Victory in the Mexican–American War results in the 1848 Mexican Cession of California and much of the present-day American Southwest.

The California Gold Rush of 1848–49 spurs migration to the Pacific coast, which leads to the California Genocide and the creation of additional western states.

Manifest Destiny is the belief that American settlers are destined to expand across the continent.

Manifest Destiny is rejected by modernizers, especially the Whigs like Henry Clay and Abraham Lincoln who want to build cities and factories—not more farms.

Democrats strongly favor expansion, and win the key election of 1844.

After a bitter debate in Congress the Republic of Texas is annexed in 1845, leading to war with Mexico, who consider Texas to be a part of Mexico due to the large numbers of Mexican settlers.

The Mexican–American War (1846–48) breaks out with the Whigs opposed to the war, and the Democrats supporting the war.

The U.S. army, using regulars and large numbers of volunteers, defeats the Mexican armies, invades at several points, captures Mexico City and wins decisively.

The Treaty of Guadalupe Hidalgo ends the war in 1848.

Many Democrats want  to annex all of Mexico, but that idea is rejected by southerners who argue that by incorporating millions of Mexican people, mainly of mixed race, would undermine the United States as an exclusively white republic.

Instead the U.S. takes Texas and the lightly settled northern parts (California and New Mexico)

The Hispanic residents are given full citizenship and the Mexican Indians become American Indians.

Simultaneously, gold is discovered in California in 1849, attracting over one hundred thousand men to northern California in a matter of months in the California Gold Rush.

A peaceful compromise with Britain gives the U.S. ownership of the Oregon Country, which is renamed the Oregon Territory.

The issue of slavery in the new territories is seemingly settled by the Compromise of 1850, brokered by Whig Henry Clay and Democrat Stephen Douglas; the Compromise includes the admission of California as a free state in exchange for no federal restrictions on slavery placed on Utah or New Mexico.

The point of contention is the Fugitive Slave Act, which increases federal enforcement and requires even free states to cooperate in turning over fugitive slaves to their owners

Abolitionists pounce on the Act to attack slavery, as in the best-selling anti-slavery novel Uncle Tom's Cabin by Harriet Beecher Stowe.

Further aggravating the dispute is the fact that the Texans have issued a dubious territorial claim that expands the republic's southern and western boundary from the previously accepted Nueces River to the Rio Bravo del Norte.

By claiming all of the land up to the headwaters of the Rio Bravo del Norte, the Texans have more than double the size of their republic to include parts of present-day New Mexico, Colorado, Oklahoma, Kansas, and all of present-day western Texas.

The situation in Mexico City is chaotic.

Antonio López de Santa Anna, president once again, denounces both congress and his own subordinates in the executive branch for their lack of resolve in preparing the defense of the capital.

They, in turn, denounce him for his failures in battle.

On August 20, 1847, the Army of Occupation asks for the surrender of Mexico City, but the battle continues until September 13, 1847, when the last bastion of Mexican resistance falls during the famous Battle of Chapultepec.

During the battle, young cadets from the Mexican military academy, the Niños Héroes (or "boy heroes") leap to their deaths rather than surrender.

The United States victory marks the end of the war and the beginning of negotiations for peace.

The heaviest fighting is done by General Winfield Scott's Army of Occupation, which lands at Veracruz on March 9, 1847.

Rather than attempt to occupy the city outright, Scott positions his forces west of it, cutting off Veracruz's supply line from the capital.

After several days of heavy naval bombardment that kills hundreds of civilians, Veracruz surrenders on March 27, 1847.

The United States Army attacks Mexico on three fronts: one column, under General Stephen W. Kearney, occupies California and New Mexico; another column, under General Zachary Taylor, enters northern Mexico; and a third detachment, commanded by General Winfield Scott, lands at Veracruz and marches to Mexico City.

California and New Mexico fall with little bloodshed.

Northern Mexico is the scene of fierce battles between Taylor and Santa Anna's armies at Buena Vista.

Santa Anna initially strikes hard at the outnumbered United States forces, but he later abandons the battle and returns to Mexico City, prematurely claiming victory.

Hostilities between Mexico and the United States begin on April 25, 1846, when several United States soldiers are killed in a cavalry skirmish with Mexican forces in the disputed territory.

Shortly after the two sides declare war, Santa Anna is recalled from exile in Cuba to once again lead Mexican troops against a foreign invasion.

U.S. President James K. Polk dispatches a special envoy, John Slidell, to Mexico City shortly after Texas is admitted to the Union as the twenty-eighth state, to settle the Texas boundary dispute and to arrange the purchase of California.

The Mexican president, José Joaquín de Herrera, had been willing to recognize an independent Texas but is under intense domestic pressure to reject United States annexation and Texas's expanded territorial claim.

As a result, he refuses to meet Slidell and begins reinforcing Mexican army units along the Rio Bravo del Norte.