The Mexican-American War of 1846-1848 was the armed manifestation of the doctrine of Manifest Destiny that encouraged American expansionism in the 19th century. While the conflict is undoubtedly eclipsed in historical memory by the American Civil War fought less than two decades later, the Mexican-American War transformed the map of the United States and altered the countrys relationship with its southern neighbor. Moreover, the conflict was the stage for the rise of influential political and military leaders who fought on both sides of the American Civil War.CausesPresident James K. Polk by Charles Fenderich, 1838. Source: National Portrait Gallery, Smithsonian InstitutionIn 1846, the Mexican-American War broke out following a series of territorial disputes fueled by American expansionism. Manifest Destiny, the idea that the United States was inherently fated to spread democracy and capitalism across North America, guided American political and military ambitions during the mid-19th century. Led by President James K. Polk, the United States sought to bring territory in modern-day Texas, California, and New Mexico under American control.One of the primary catalysts of the Mexican-American War were the disputes over the possession of Texas. In 1836, the Republic of Texas gained its independence from Mexico after a revolution driven by violent clashes including the 13-day siege of the Alamo. After achieving autonomy, most Texans favored rapid annexation into the United States. The formal takeover process, however, was slowed by American disagreements over the status of slavery in the new territory.By 1845, the Republic of Texas was annexed by the United States. This provoked a hostile reaction from Mexican political leaders, who continued to lay claim on the territory. Furthermore, the two parties disagreed about the boundaries of Texas. While the United States asserted the Rio Grande marked its southern border, Mexican authorities placed Texass boundary 100 miles to the north near the Nueces River. Before long, President Polk sent American troops to lay claim to the disputed territory.Failed Negotiations and Early ClashesGeneral Zachary Taylor by Jacques Reich, 1899-1920. Source: National Portrait Gallery, Smithsonian InstitutionBefore deploying soldiers near the disputed Mexico-Texas border, President Polk first attempted to advance American political goals through diplomatic methods. Sending American lawyer and future senator John Slidell as a special envoy to Mexico City, Polk planned to negotiate the purchase of California and New Mexico for $25 million. Mexico, however, was plagued by internal political instability and rejected the offer altogether by declining to meet with Slidell.On April 25, 1846, the Mexican-American War formally commenced when Mexican forces engaged US troops in the contested region between the Nueces and Rio Grande Rivers. After successfully provoking a military response from Mexico, Polk secured a formal declaration of war from Congress less than three weeks later on May 13.The first significant engagement of the Mexican-American War, the Battle of Palo Alto, occurred on May 8, 1846. There, future president General Zachary Taylor led American troops against General Mariano Aristas Mexican forces. Despite being outnumbered by nearly 1,400 soldiers, the Taylor achieved a major victory by exploiting strategic advantages in its mobile light artillery, boosting morale for the battles to come.Other Major BattlesJohn E. Wool on horseback by Ehrgott & Forbriger Lithography Company, 1862-1864. Source: National Portrait Gallery, Smithsonian InstitutionThe Mexican-American War witnessed further major engagements in 1847. In February, Generals Zachary Taylor and John E. Wool led United States troops in the Battle of Buena Vista against General Antonio Lpez de Santa Annas forces. In the face of an overwhelming Mexican force, Taylor and Wood deployed their forces at a narrow pass, concentrating their artillery towards the enemys pointed assaults. After several attempts by the Mexicans to break through, the Americans successfully defended Buena Vista, forcing the Mexicans to fall back. While both sides claimed victory, Taylors successful defense further enhanced his image in the public eye, boosting his presidential bid two years later.In March, General Winfield Scott led the United States military in its first large-scale amphibious assault in American history at the Siege of Veracruz. After landing his troops at Collado beachheads south of the objective, the two sides engaged in 20 days of intense fighting. Scotts superior tactics and weaponry outperformed that of Mexicos General Juan Morales. The occupation of Vera Cruz allowed Scott to use its port as a supply hub for operations further inland.General Winfield Scott, distributed by D. Needham, 1847. Source: National Museum of American History, Smithsonian InstitutionGeneral Scott launched an ambitious offensive targeted at the Mexican capital, and by September 1847 American forces approached the Castle of Chapultepec, a key military fortification and training complex in Mexico City. Scotts artillerymen launched an effective bombardment of the target before ordering infantrymen to scale the castles walls and capture the stronghold. This bold strategy enabled the Americans to occupy Mexico City and bring an end to the Mexican-American War in early 1848.Key FiguresMexican-American War Generals Harvey, Wool and Mansfield by John Henry Bufford, 1861. Source: National Portrait Gallery, Smithsonian InstitutionThe Mexican-American War paved the way for prominent military and political leaders to thrive in the public eye. Victory in the Mexican-American War enabled President Polk to achieve all of his political objectives. After Polk honored his pledge to stand down after a single term, Zachary Taylor contested the 1848 election as the Whig Party candidate and defeated Democrat Lewis Cass of Michigan. Taylor died in 1850 after two years in office, and his colleague Winfield Scott led the Whig ticket in the 1852 election. Minister of War General Juan Almonte, who organized the defense of Mexico City, would eventually become a leading figure in the Second Mexican Empire.The relatively short conflict also gave opportunities for future commanders of the American Civil War to prove themselves in combat. Then-Captain Ulysses S. Grant served under Scott during the siege of Mexico City, playing a key role in creatively employing artillery to overcome restrictive terrain near Chapultepec Castle. This experience contributed to his reputation as a resourceful officer and empowered his rise as the Unions Commanding General and 18th American President.At the battle of Cerro Gordo in April 1847, Grant fought alongside future Confederate generals Robert E. Lee and Pierre G. T. Beauregard. The latter would command Confederate forces at the Battle of Fort Sumter and the First Battle of Bull Run, while the former served as commander of the Army of Northern Virginia and eventually surrendered to Grant at Appomattox in April 1865. Other famous figures of the American Civil War who served in the Mexican-American War include George McClellan, Thomas Stonewall Jackson, William Tecumseh Sherman, and George Meade.OutcomesTaylors victories on the Rio Grande, artist unknown, 1846. Source: National Museum of American History, Smithsonian InstitutionThe signing of the Treaty of Guadalupe Hidalgo on February 2, 1848 marked the end of two years of hostilities between the United States and Mexico. As the victorious power, the United States more than satisfied its prewar expansionist desires by acquiring over 500,000 square miles of land, increasing the size of the country by nearly one-third. Lands ceded by Mexico after the war include present-day California, Nevada, and Utah in addition to parts of Wyoming, Colorado, and Arizona. The United States, in return, compensated Mexico with $15 million while charging their southern neighbor with $3.25 million in debts owed to America.A major impact of the Mexican-American War was the uncertainty surrounding the status of slavery in Americas newly acquired territories. Abolitionists supported the proposed Wilmot Proviso, which set out to prohibit slavery in states attained via the Treaty of Guadalupe Hidalgo, but the resolution was never passed by Congress. Continued disagreements about slavery would contribute to the outbreak of the civil war over a decade later.For Mexico, the wars outcomes were devastating. In addition to ceding a vast amount of territory and recognizing the US annexation of Texas, the Mexican-American War exacerbated the countrys economic and political volatility. Thousands of Mexican citizens were left displaced in the territories yielded to the United States, leaving families to hastily choose to migrate deeper south into Mexico or accept residence in what had previously been a foreign land.Lasting Effects and LegacyGeneral Ulysses S. Grant, date unknown. Source: National Museum of American History, Smithsonian InstitutionIn addition to the short-term impacts which led to the Civil War, the conflict had long-term effects that can still be observed today. The once-disputed border region of Texas continues to present opportunities and challenges for each country. Illegal immigration, border security, and drug trafficking in the region are controversial topics that motivate national political discourse in the United States.From Mexicos perspective, the conflict inspires modern interpretations of historical grievances against the United States. While both sides played a role in instigating the war, the massive territorial gains by the United States contributed to decades of national trauma and complicated political bonds between the two countries. American territories acquired via the Treaty of Guadalupe Hidalgo such as California and Arizona now make a major contribution to Americas economy, amplifying the sense of loss in Mexico.There is little doubt that the Mexican-American War, while largely overlooked in the present day American history, had a major impact on both countries. Much like improved Anglo-American relations following the War of 1812, the United States and Mexico are fortunate to remedy historical differences to arrive at mostly positive political, military, and economic relationships between the two North American neighbors today.