During his time in office, President James K. Polk — a firm believer in the idea of “Manifest Destiny” — oversaw the most significant territorial expansion in the history of the United States. The expansion was largely achieved through three events:
- The Annexation of Texas in 1845.
- The Oregon Treaty of 1846.
- The Mexican-American War.
Collectively, these events led to the United States gaining control over the future states of Texas, California, Nevada, New Mexico, Arizona, Utah, Washington, and Oregon, as well as portions of what would later become Oklahoma, Colorado, Kansas, Wyoming, and Montana.
Of the three events, the Mexican-American War was the most controversial and came with significant ramifications.
The Annexation of Texas Sets the Stage for the Mexican-American War
In the aftermath of the Texas Revolution (1835–1836), Mexico threatened to declare war on the United States if it annexed Texas. Mexican officials were certain the United States had played a role in inciting the revolution and intended to annex the territory.
Although there was support for adding Texas to the Union at the time, both President Andrew Jackson and President Martin Van Buren refrained from doing so. Neither of them wanted to engage Mexico. However, the United States did officially recognize Texas and carried on diplomatic relations.
In 1844, President John Tyler restarted negotiations with Texas regarding annexation. On April 12, the Texas Treaty of Annexation was agreed to, and Mexico responded by severing diplomatic relations with the United States. However, Tyler was unable to gain the support of the Senate, which defeated the treaty in June.
Tyler subsequently lost the Presidential Election of 1844 to Democrat and pro-expansion candidate, James K. Polk from Tennessee. Polk was a staunch believer in Manifest Destiny — the idea the United States had a divine right to expand westward across the continent from sea to sea.
Before he left office, Tyler tried again to annex Texas, but through a joint resolution of Congress. With Polk’s support, Tyler was able to convince Congress to pass the Tyler-Texas Treaty on March 1, 1845. Texas was admitted to the Union on December 29, 1845, as the 28th state.
Despite the annexation of Texas, Mexico did not follow through with its threat to declare war. However, the tension between the two nations increased due to a border dispute between Mexico disputed and Texas.
According to Texas, its territory included significant portions of what is today New Mexico and Colorado, along with the western and southern portions of Texas itself. Texas claimed its territory extended south to the Rio Grande River. However, Mexico insisted the border only extended to the Nueces River, which is north of the Rio Grande, and severed diplomatic relations with the United States.
Taylor Moves into the Disputed Territory on the Rio Grande River
In July 1845, President Polk ordered the commander of the U.S. Army in Texas, General Zachary Taylor, to move into the disputed territory between the Nueces River and the Rio Grande River.
The following November, Polk sent Congressman John Slidell to Mexico to negotiate with the Mexican government and purchase:
- The disputed areas along the Texas-Mexican border.
- The territory that makes up the present-day states of New Mexico and California.
However, the Mexican Government rejected Polk’s offer.
The United States Declares War on Mexico
In May 1846, news of skirmishes between Taylor’s men and Mexican troops reached Washington. Polk used it to his advantage and convinced Congress to support a Declaration of War against Mexico. However, Polk failed to mention that Taylor’s men were on the banks of the Rio Grande River — which Mexico claimed as its territory. On May 13, 1846, the United States declared war on Mexico.
Trist Negotiates the Treaty of Guadalupe-Hidalgo
Following the capture of Mexico City in September 1847, Nicholas Trist, chief clerk of the Department of State and Polk’s emissary to the Mexican Government started negotiations for a peace treaty. The terms he offered were similar to what Slidell presented om 1846.
As the Presidential Election of 1848 came closer, Polk grew concerned with Trists’ conduct and public support for the war. First, Polk was worried Trist would not press the Mexicans for terms that favored the United States. Second, Trist was an ally of General Winfield Scott, a Whig who was believed to be a contender for the party’s presidential nomination. Polk recalled Trist to Washington in October.
However, Trist felt he was close to an agreement with the Mexicans, so he ignored the recall order. He continued to negotiate and agreed to the Treaty of Guadalupe Hidalgo, which was signed in Mexico City on February 2, 1848.
Under the terms of the treaty, Mexico ceded approximately 525,000 square miles — roughly 55% of its territory — to the United States in exchange for:
- The United States agreed to pay a lump sum of $15 million to Mexico.
- The United States agreed to assume $3.25 million that Mexico owed U.S. citizens.
Although Polk and others involved in the “All of Mexico Movement” would have preferred to annex more Mexican territory, he realized that prolonging the war would have disastrous political consequences. As a result, Polk submitted the treaty to the Senate for ratification. On March 10, 1848, the Senate approved the Treaty of Guadalupe Hidalgo by a vote of 38 to 14.
The Wilmot-Proviso Bans Slavery in the New Territory
The war had significant implications beyond the expansion of the United States. On August 8, 1846, Congressman David Wilmot introduced a rider to an appropriations bill that stipulated that “neither slavery nor involuntary servitude shall ever exist” in any territory acquired by the United States in the war against Mexico.
Southern senators managed to block the adoption of the so-called “Wilmot Proviso,” but it still caused political controversy. The question of whether slavery could expand throughout the United States continued to fester until the defeat of the Confederacy in 1865, which ended the Civil War.
Mexican-American War — Manifest Destiny and Mexican Independence
1820 — Congress approves the Missouri Compromise. It outlaws slavery in the Louisiana Territory north of the 36°30′ latitude. Missouri enters the Union as a Slave State, while Maine enters as a Free State.
1820–50 — An estimated four million settlers move into the western parts of North America.
1821 — Mexico gains its independence from Spain. This allows the establishment of the Santa Fe Trail, enabling trade between the United States and Mexico. At the same time, the Mexican government grants Moses Austin land in Tejas y Coahuila — later called Texas. Austin intends to establish a colony there, with roughly 300 families. However, Austin unexpectedly dies and his son Stephen takes over for his father.
1824 — Mexico passes a constitution that establishes the nation as a republic.
Mexican-American War — Texas Revolution
1829 — Mexico bans slavery, but most Texans ignore this law as well as others that restrict new immigration and gun registration.
1833 — General Antonio López de Santa Anna becomes President of Mexico as well as supreme commander of the nation’s army. He abolishes the 1824 Constitution and sets up a centralized government, providing him with absolute power.
1833 — Stephen F. Austin is accused of inciting Texans to revolt against Mexico. He is arrested and imprisoned.
1835 — Santa Anna sends troops to Texas. Texans resist and take control of the towns of Gonzales, Goliad, and San Antonio.
March 2, 1836 — At Washington-on-the-Brazos, Texans establish the independent Republic of Texas. David Burnet is elected interim President, and Sam Houston is given command of the Texas Army.
March 6, 1836 — Battle of the Alamo — A large Mexican force under Santa Anna attacks less than two hundred Texans holed up in the Alamo, an old Spanish mission at San Antonio. All of the Alamo’s defenders are killed.
March 19, 1836 — After losing a battle against the Mexican army at Goliad, three hundred Texan soldiers under the command of Colonel James Fannin are taken as prisoners.
March 27. 1936 — Goliad Massacre — Santa Anna orders Fannin and the others to be executed. The massacres at Goliad and the Alamo enrage Texans, and the small Texan army expands as men volunteer to join the fight.
April 21, 1836 — Battle of San Jacinto — Sam Houston leads 900 Texans in a surprise attack against several thousand Mexican troops under Santa Anna’s command. The Battle of San Jacinto is a victory for the Texans, who suffered only 40 casualties to the Mexicans’ 600 killed and 730 captured. Santa Anna is captured on the 22nd and eventually forced to sign a treaty recognizing the independence of Texas. The treaty is later considered invalid by the Mexican Government.
October 1836 — Sam Houston becomes the first elected president of the Republic of Texas.
Mexican-American War — Texas Annexation
1837 — President Andrew Jackson recognizes Texas as an independent republic. Jackson wants to annex Texas, but his efforts are for unsuccessful.
1843 — Mexico warns the United States it will declare war if it annexes Texas.
1844 — James K. Polk, who believes in America’s Manifest Destiny, is elected President of the United States.
1845 — In an article published in the Democratic Review, newspaper editor John O’Sullivan coins the term “Manifest Destiny,” referring to the idea that Americans have a divine right to spread across the continent, from the East Coast to the West Coast.
March 1, 1845 — The United States annexes Texas. In protest, Mexico cuts off diplomatic relations with the United States.
March 4, 1845 — James K. Polk is inaugurated President of the United States. Soon after, he orders General Zachary Taylor to move into the disputed territory between the United States and Mexico.
June 16, 1845 — The United States officially annexes Texas.
Summer 1845 — President Polk orders General Zachary Taylor to take several thousand U.S. troops to Corpus Christi on the Nueces River, which is recognized as the traditional border between Texas and Mexico.
November 1845 — Polk sends diplomat John Slidell to Mexico to offer its government $5 million for New Mexico and $25 million for California. Mexico rejects the offer.
March 28, 1846 — Taylor’s troops move several hundred miles south to the banks of the Rio Grande, which the United States claims as the border with Mexico. On a site across from the Mexican city of Matamoros, Taylor begins construction on Fort Texas — later renamed Fort Brown.
April 11, 1846 — Mexican General Mariano Arista arrives at Matamoros and takes charge of the Mexican troops stationed there.
Mexican-American War Begins
April 25–26, 1846 — Thornton Affair — A small U.S. patrol unit of 63 soldiers is attacked by a much larger Mexican force. Eleven soldiers are killed. Taylor informs Polk that hostilities have begun. Following the skirmish, Mexican forces lay siege to Fort Texas.
May 8, 1846 — Battle of Palo Alto — American forces led by Taylor engage Mexican forces. The Mexicans withdraw that night, and Taylor pursues them.
May 9, 1846 — Battle of Resaca de la Palma — Taylor’s troops defeat the Mexican army for the second day in a row. The Mexicans suffer heavy casualties. During the course of the two days, the weakness of the Mexican artillery is exposed.
May 11, 1846 — Declaration of War — President Polk sends a Declaration of War to Congress, claiming Mexico has started the war by shedding “American blood upon American soil.”
May 13, 1846 — Declaration of War — Congress declares war on Mexico.
May 18, 1846 — General Taylor and his army occupy Matamoros after Mexican troops evacuate and retreat to Monterrey.
June 1846 — Bear Flag Revolt — Led by Colonel John Charles Frémont of the U.S. Corps of Topographical Engineers, a group of American settlers in California revolts against Mexico, establishing the Bear Flag Republic.
July 1846 — New England philosopher and writer Henry David Thoreau, vowing that his money will not go to support the war in Mexico, is arrested and jailed for refusing to pay his taxes. The experience inspires him to write his famous essay “Civil Disobedience” regarding what he considers are the duties of a good citizen.
Summer 1846 — Thousands of American volunteers arrive in the lower Rio Grande Valley, joining Taylor’s Army of Occupation. It is estimated that 75,000 volunteers enlist, so a draft is unnecessary.
August 13, 1846 — Combined Navy and Army forces under the command of Commodore Robert Stockton take control of Los Angeles, ending a series of peaceful takeovers of towns running along the coast of California
August 15, 1846 — Capture of Santa Fe — General Stephen Watts Kearny arrives in Santa Fe at the head of the U.S. Army of the West. The town is captured bloodlessly, and Kearny soon sets off for California with several hundred troops.
September 1846 — Santa Anna returns to Mexico from Cuba, where he had been in exile. He starts building up his army to engage the Americans.
September 21–25, 1846 — Battle of Monterrey — Taylor’s forces take control of the city of Monterrey after a bloody battle that ends with fierce hand-to-hand fighting. Taylor negotiates an 8-week ceasefire, which upsets President Polk.
September 29, 1846 — Siege of Los Angeles — General José María Flores leads a small Mexican force in recapturing Los Angeles.
November 1846 — Polk offers General Winfield Scott command of the Army of Invasion. Scott plans an amphibious attack — using the combined forces of the Army and Navy — to be launched from the coastal city of Veracruz. From there, Scott intends to lead his forces west toward the capital, Mexico City.
December 1846 — Santa Anna is elected President of Mexico.
December 6, 1846 — Battle of San Pascual — Exhausted by their journey from Santa Fe to California, Kearny, and his men lose the Battle of San Pascual.
January 8, 1847 — Battle of Rio San Gabriel — The combined forces of Stockton, Frémont, and Kearny reconquer Los Angeles. As a result, California is controlled by the United States.
February 22–23, 1847 — Battle of Buena Vista — Taylor’s army meets Santa Anna’s army near the Buena Vista ranch, roughly 45 miles west of Monterrey. After a long day of fighting and an estimated 3,000 casualties, the Mexicans retreat.
February 28, 1847 — Battle of the Sacramento River — Led by Colonel Alexander Doniphan, American forces take control of Chihuahua.
March 22, 1847 — Siege of Veracruz — Having landed more than 10,000 troops near Veracruz, General Scott bombards the city. Almost 200 Mexicans — half of them civilians — are killed in the attack. The city comes under U.S. control on March 28.
April 18, 1847 — Battle of Cerro Gordo — A short battle at a mountain pass called Cerro Gordo sends the Mexican army fleeing after around 1,000 of them are killed or wounded, and another 3,000 are captured.
May 1847 — Spanish-speaking diplomat Nicholas Trist joins General Scott’s army at Puebla. Trist has been sent to negotiate a peace treaty with the Mexicans. Although Trist clashes with Scott at first, the two eventually become friends. Trist negotiates with the Mexican Government but is unsuccessful.
August 11, 1847 — Scott’s army enters the Valley of Mexico. The assault on Mexico City will take place over the next month, with a series of battles culminating in a U.S. victory.
August 19 and 20, 1847 — Battle of Contreras and the Battle of Churubusco — On the outskirts of Mexico City, the Mexicans sustain about 4,000 casualties, while the United States has nearly 1,000. The surviving 75 members of the San Patricio Battalion, a Mexican army unit made up of U.S. Army deserters and other foreigners, are captured; 50 are sentenced to death by hanging while the rest are to be severely punished.
August 21, 1847 — An armistice takes effect and the two sides enter into peace talks. Each determines that the other is simply stalling for time, and the armistice is called off on September 7.
September 8, 1847 —Battle of Molino del Rey — U.S. forces attack Fort El Molino del Rey, where they believe the Mexicans are manufacturing cannons. The U.S. forces sustain their highest casualties for a single battle of the war — 23 percent of those participating — and no cannon factory is found.
September 13, 1847 — Battle of Chapultepec — The United States attacks Chapultepec Hill, the site of Mexico’s National Military Academy and the last obstacle before Mexico City. Nearly all 800 Mexican defenders are killed, including 50 cadets from the Academy who will be remembered as Los Ninos Heroes — The Boy Heroes.
September 14–15, 1847 — Battle for Mexico City — After a bloody two-day assault, the United States takes control of Mexico City. President Santa Anna flees and the Mexican government is in disarray.
October 11, 1847 — U.S. forces broke the Siege of Puebla.
October 19–20, 1847 — Battle of Guaymas — U.S. Naval forces capture the port town of Guaymas.
November 11, 1847 — A new Mexican Congress is formed and an interim President, Manuel de la Pena y Pena, is elected. However, the movement toward peace negotiations is slow.
November 16, 1847 — Trist receives a message from Polk, ordering him to return to the United States. However, Trist is convinced that the opportunity for peace will be lost, and he decides to defy the order and enter into peace talks.
Mexican-American War Ends
January 2, 1848 — Trist begins secret negotiations at Guadalupe Hidalgo with three Mexican peace commissioners. They are able to agree to terms.
February 2, 1848 — The Treaty of Guadalupe Hidalgo is signed. Mexico agrees to recognize the Rio Grande border and to cede California and New Mexico to the United States. The United States agrees to pay Mexico $15 million and forgive all debts owed to American citizens. Native Mexicans living in the territories are given the option to become U.S. citizens.
March 6, 1848 — Truce of March 6 — A truce is declared, which is supposed to end hostilities. However, Mexican partisans continue to attack the U.S. Army of Occupation.
March 10, 1848 — The Senate approves the Treaty of Guadalupe Hidalgo.
March 16, 1848 — President Polk signs the Treaty of Guadalupe Hidalgo.
March 16, 1848 — Battle of Santa Cruz de Rosales — American forces led by Sterling Price advance into Chihuahua. Price occupies Chihuahua and engages Mexican forces after being told by the Mexican Governor Ángel Triás that the Truce of March 6 was already signed
May 30, 1848 — The Mexican Government ratifies the treaty, making it official.
July 4, 1848 — The United States receives its copy of the signed treaty.
Mexican-American War — Aftermath
January 1849 — California Gold Rush — Gold is discovered at Sutter’s Mill, near the present-day city of Sacramento, California. Inspired by the prospect of instant wealth, many Americans move to the West and in the Gold Rush.
1850 — Compromise of 1850 — Tensions over slavery that had been intensified by the addition of new territories to the United States after the Mexican-American War are temporarily eased by the Compromise of 1850. The compromise admits California as a Free State, with both slaveholders and others allowed to settle in New Mexico and Utah. Slavery is abolished in the District of Columbia, but the Fugitive Slave Law is to be more strictly enforced.
1861–65 — American Civil War — Eleven states secede from the Union to form the Confederate States of America, and the nation is engulfed in a bloody civil war over the issues of slavery, taxation, and state’s rights. The war ends with the defeat of the Confederacy, and the restoration of the Union, but has long-lasting effects on the nation.