After the United States secured independence in the 1783 Treaty of Paris, no peace treaty of the 19th century proved as important to the countrys early history than the Treaty of Guadalupe Hidalgo. While the Treaty of Ghent that ended the War of 1812 three decades earlier failed to address the conflicts key causes, the decisive American victory in the Mexican-American War ensured that the Treaty of Guadalupe Hidalgo resolved most of the prewar disputes with Mexico in Americas favor.The Causes of the WarPresident James Polk by Nathaniel Currier, 1846. Source: Smithsonian InstitutionThe Mexican-American War arose from a number of interrelated causes, most of which were subsequently addressed by the Treaty of Guadalupe Hidalgo. While the fighting broke out on April 25, 1846 after Mexican troops ambushed American soldiers in the disputed region around Texas southern border between the Rio Grande and Nueces Rivers, the conflict had longer term catalysts. The United States annexation of Texas in 1845 disgruntled its southern neighbor, which continued to regard Texas as Mexican territory. Following the annexation, the two countries disagreed on the borders of Texas, prompting American President James Polk to send a special envoy to Mexico.Seeking to deescalate border tensions and armed with a proposal to acquire New Mexico and California from Mexico for $30 million, American lawyer and future senator John Slidell went to Mexico City but was outright ignored by the Mexican government. In response, President Polk mobilized American troops to the disputed region. As Polk had anticipated, his decision triggered a Mexican military response and allowed him to secure congressional authorization for war under the pretext of self-defense.Some 15 months later, on September 14, 1847, the United States captured Mexico City, ending major hostilities of the conflict after key engagements including the Battles of Palo Alto, Monterrey, Buena Vista, and Veracruz led by American generals Zachary Taylor and Winfield Scott. After several months of negotiations, the Treaty of Guadalupe Hidalgo was concluded in February 1848.Signatories and TermsThe final page of the Treaty of Guadalupe Hidalgo, 1848. Source: Library of CongressRepresentatives from the belligerent parties met in the town of Guadalupe Hidalgo, now a neighborhood of Mexico City, to discuss peace terms to end the war. Nicholas Trist, chief clerk of the State Department, was the primary representative and sole signer for the Americans. The Mexican diplomats Luis Cuevas, Bernardo Couto, and Miguel Atristain represented the defeated party.During the negotiations, Trist offered more generous terms to Mexico than Polk had anticipated, enabling the Mexicans to retain the whole of Baja California. This prompted Polk to recall Trist before negotiations ended, but Trist ignored the order and concluded the treaty on February 2, 1848. Trist was fired upon his return to Washington.The most consequential articles of the Treaty of Guadalupe Hidalgo involved the cession of over half a million square miles of Mexican territory to the United States. The agreement also settled the disputes involving Texas as Mexico renounced its claim to Texas and officially recognized the Rio Grande as the states southern border.While these results were devastating to Mexico, the United States agreed to compensate the country with $15 million for its newly acquired lands, offset by the payment of just over $3 million in debt owed to American citizens. Lastly, the agreement gave Mexican citizens living in regions transferred into American ownership the option of remaining in their homes as full American citizens.Impact on MexicoPortrait of Emperor Maximilian by Andrew Burgess, 1864. Source: Smithsonian InstitutionThe Mexican-American War and resulting Treaty of Guadalupe Hidalgo plummeted Mexico into political and economic strife in the decades following the conflict. Its surrender of 525,000 square miles of land to the United States was a devastating blow to Mexicos territorial integrity and national pride after its independence from Spain. Mexico lost important resources including agricultural land and valuable minerals. In a further blow to Mexican pride, gold and oil were discovered in the ceded territories after the treaty was signed.The Mexican-American War left Mexico deeply indebted to foreign powers including France, Spain, and Britain. In 1861, Mexican President Benito Jurez cancelled national debt payments. To collect its debts and counter American influence in North America, the three European nations formed a tripartite alliance in Mexico in hopes of establishing a conservative government that would honor Mexicos debts. Spain and Britain ultimately withdrew from the pact, leaving Napoleon IIIs France alone in its Mexican ambitions. In 1863, France seized Mexico City and installed Archduke Maximilian of Austria as emperor. Preoccupied with the American Civil War, the United States was unable to intervene and enforce the Monroe Doctrine until the end of the war.Emperor Maximilian was finally overthrown, put on trial, and executed in 1867, and Jurez returned to power as president. By then, however, long-term damage was done. The political and economic turmoil in the decades following the Treaty of Guadalupe Hidalgo further delayed Mexicos recovery after its calamitous defeat to the United States.Impact on the United StatesIllustration of the Mexican Cession of 1848 under the Treaty of Guadalupe Hidalgo. Source: Wikimedia CommonsThe Treaty of Guadalupe Hidalgo naturally favored the wars victor, but its impact on the United States was by no means exclusively positive. The United States acquired territory encompassing the states of California, Nevada, Utah, Arizona, New Mexico, Colorado, Wyoming, and Texas. These territorial acquisitions largely fulfilled Americas expansionist aims driven by a belief in the countrys Manifest Destiny to dominate the North American continent from coast to coast.In the decades following their acquisition, the new territories proved economically fruitful for the United States. The California Gold Rush of 1849 rapidly encouraged settlement in the American West, while other states served as mining, agricultural, and transportation centers that bolstered industrialization in the second half of the 19th century. Disagreements surrounding the status of slavery in new lands, however, contributed to sectional tensions in the United States following the Mexican-American War.As an international agreement between the United States and Mexico, the Treaty of Guadalupe Hidalgo did not address the question of slavery in the territories acquired by the United States. While Congressman David Wilmot of Pennsylvania proposed a ban on slavery in the new territories, the proposal was passed by the House but defeated in the Senate. Sectional disagreements on whether California should be admitted to the Union as a slave state or a free state led to the Compromise of 1850 and the rise of the new antislavery Republican Party. The sectional tensions over slavery eventually contributed to the American Civil War, a destructive conflict that ultimately resulted in the abolition of slavery.Impact on Native AmericansApache Chief Natchez Nei-Chi-Ri in Arizona, 1885. Source: Smithsonian InstitutionAside from its effects on Mexico, the Treaty of Guadalupe Hidalgo was especially detrimental to Native American populations living within the ceded territories. Reorganization of North American lands led to the displacement of numerous tribes in the American West, specifically the Apache, Comanche, and Navajo. Territorial disputes often led to violent clashes between Native Americans and American settlers, as well as regular US Army units. Direct and indirect outcomes for Native Americans after the Mexican-American War include the Navajo Long Walk, which left thousands dead, and the Apache Wars between the United States Army and Chiricahua and Western Apache groups.Article XI of the Treaty of Guadalupe Hidalgo explicitly placed responsibility on the United States to protect both Mexican and American citizens from Native American raids. Although the agreement also stated that special care shall then be taken not to place its Indian occupants under the necessity of seeking new homes, this clause did little to prevent the ensuing hostilities.Now under protection of the United States, Mexican families who chose to remain in their homes were vulnerable to ambushes by Native Americans, and resulting efforts to resolve the conflicts between settlers and Native Americans were frequently ineffective and unfavorable to Native Americans. The Kiowa and Comanche Treaties, for instance, forced Native American groups to forfeit land in modern-day Texas. In many cases, the United States violated the terms of their agreements with Native American populations, leading to the large-scale dehumanization of tribal peoples in the nineteenth century after the Mexican-American War.Issues With the Treaty and the Gadsden PurchasePortrait miniature of James Gadsden by Charles Fraser, c. 1831. Source: Gibbes Museum of Art, Charleston, South CarolinaThe Treaty of Guadalupe Hidalgo, like most peace treaties, was an imperfect document. Aside from American struggles to protect Mexican citizens from Native American ambushes, the agreement was based on an erroneous map that led to further border disputes to the west of the Rio Grande.The disputed territory claimed by the United States following a survey included the Mesilla Valley in southern New Mexico, which was regarded as an optimal route for a transcontinental railway line. The dispute led to further negotiations that resulted in the Gadsden Purchase of 1853, brokered between American railroad executive and diplomat James Gadsden and President Santa Anna of Mexico.The acquisition of a strip of land in what is now southern New Mexico and Arizona via the Gadsden Purchase addressed the ambiguities in the Treaty of Guadalupe Hidalgo and enabled the United States to complete the construction of the transcontinental railroad in 1881. In return, Mexico, severely indebted to European powers and struggling to recover economically, received $10 million from the United States.Despite its flaws, the Treaty of Guadalupe Hidalgo fulfilled American expansionist visions and the newly acquired territories have made significant contributions to the American economy. However, with the benefit of hindsight, it is imperative to bear in mind the agreements negative effects on Mexico and Native Americans. These negative impacts have been acknowledged more widely within the United States in recent decades.