The Mexican-American War, a significant conflict that reshaped the map of North America, is a subject of intense historical interest. Understanding when this war started requires delving into the complex web of events and escalating tensions between the United States and Mexico in the mid-19th century. While pinpointing a single moment can be nuanced, examining the key events provides a clear picture of the war’s commencement.
Background to the Mexican-American War
The roots of the Mexican-American War are multifaceted, stemming from territorial ambitions, political ideologies, and unresolved disputes. A crucial precursor was Texas’s independence from Mexico in 1836. The newly formed Republic of Texas sought annexation by the United States, a proposition initially met with hesitation in Washington. Concerns over adding another slave state and the potential for conflict with Mexico, which still considered Texas as part of its territory, stalled immediate action. Mexico consistently warned that any attempt by the U.S. to annex Texas would be viewed as an act of war.
Despite these warnings, the expansionist ideology of Manifest Destiny gained considerable traction in the United States. This belief asserted the divine right and duty of the American nation to expand its dominion across the North American continent. Figures like James K. Polk, elected president in 1844, were ardent proponents of Manifest Destiny. Polk campaigned on platforms advocating for the “re-annexation” of Texas and the “re-occupation” of the Oregon Territory. His ambitions extended further, encompassing California, New Mexico, and the vast territories of the present-day American Southwest.
Polk’s desire to acquire these lands led him to offer to purchase them from Mexico. When this offer was rebuffed, he adopted a more provocative strategy. He ordered U.S. troops, under the command of General Zachary Taylor, into the territory between the Rio Grande and Nueces River. This zone was historically disputed; Mexico recognized the Nueces River as its northern border with Texas, while Texas claimed the Rio Grande. By moving troops into this contested area, Polk deliberately escalated tensions and created a flashpoint for conflict.
The Spark: April 25, 1846 – The Thornton Affair
The simmering tensions finally ignited on April 25, 1846. Mexican cavalry forces engaged a unit of U.S. soldiers in the disputed zone north of the Rio Grande. This encounter, later known as the Thornton Affair, resulted in the deaths of several American soldiers. Following this attack, Mexican forces initiated a siege of Fort Texas, a U.S. fortification situated along the Rio Grande.
General Zachary Taylor, responding to the Mexican actions, called for reinforcements. The U.S. forces, leveraging their superior weaponry, including rifles and artillery, achieved decisive victories in the ensuing battles of Palo Alto and Resaca de la Palma. These early engagements marked the first military clashes of the Mexican-American War.
The Official Start: U.S. Declaration of War on May 13, 1846
News of the Thornton Affair and the subsequent battles reached Washington D.C., prompting President Polk to address the U.S. Congress. In his message, Polk declared that Mexico had “invaded our territory and shed American blood upon American soil,” asserting that the “cup of forbearance had been exhausted.” He framed the conflict as a defensive response to Mexican aggression.
Capitalizing on the public outrage and nationalistic fervor, Polk requested a declaration of war. On May 13, 1846, the U.S. Congress officially declared war on Mexico. It is important to note that this declaration was made by the United States; Mexico never formally declared war on the U.S. Despite some opposition from northern lawmakers who questioned the motives and justification for the war, the declaration passed, marking the official commencement of the Mexican-American War from the U.S. perspective.
Early Battles and U.S. Advance into Mexico
Following the declaration of war, the United States launched a full-scale military campaign. U.S. forces, under the command of figures like Stephen Watts Kearny and Robert Field Stockton, swiftly occupied vast territories north of the Rio Grande, encountering minimal resistance due to the relatively small Mexican population in these areas.
General Taylor continued his advance into Mexico, capturing the strategic city of Monterrey in September 1846. Faced with mounting losses, Mexico recalled General Antonio López de Santa Anna from exile. Santa Anna, a cunning and opportunistic leader, initially misled Polk into believing he would negotiate a peace favorable to the U.S. However, upon his return to Mexico, Santa Anna double-crossed Polk, assuming command of the Mexican army and renewing the fight.
Despite Santa Anna’s efforts, the U.S. military advantage proved decisive. Under the leadership of General Winfield Scott, U.S. troops landed at Veracruz and marched inland towards Mexico City, mirroring the historical path of Hernán Cortés’s conquest centuries earlier. After overcoming Mexican resistance in battles like Cerro Gordo and the siege of Chapultepec Castle in Mexico City, the U.S. effectively brought the war to a close.
Treaty of Guadalupe Hidalgo and the War’s Aftermath
The Mexican-American War officially concluded with the signing of the Treaty of Guadalupe Hidalgo on February 2, 1848. This treaty established the Rio Grande as the border between the United States and Mexico, solidifying Texas’s annexation and ceding vast territories, including California and much of the present-day Southwest, to the United States for $15 million and the assumption of U.S. damage claims.
The war resulted in a massive expansion of U.S. territory, adding approximately 525,000 square miles – a landmass nearly equivalent to the Louisiana Purchase. This acquisition profoundly altered the geography, culture, and economy of the United States. However, the newly acquired territories also reignited the contentious debate over slavery, setting the stage for future internal conflict and ultimately contributing to the outbreak of the American Civil War.
In conclusion, while the Thornton Affair on April 25, 1846, marked the first armed conflict, the Mexican-American War officially started on May 13, 1846, when the United States Congress formally declared war on Mexico. This declaration followed a period of escalating tensions fueled by territorial disputes, expansionist ambitions, and ultimately, the clash in the disputed zone that served as the immediate trigger for the war.
Sources
The Mexican American War. PBS: American Experience.
The Mexican-American war in a nutshell. Constitution Daily.
The Mexican-American War. Northern Illinois University Digital Library.
