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Geography and Climate

by Richard V. Francaviglia

At the beginning of the U.S.-Mexican War, Mexico covered more than one-third of the North American continent, extending as far northwest as the present-day California-Oregon border at 42° north latitude and northeast to the Missouri River. Texas independence (1836) and statehood (1845) had substantially reduced Mexico's territory by moving Mexico's eastern border to the Nueces River, which Mexico claimed as the border, while Texas and the United States claimed the Rio Grande (or Rio Bravo del Norte) as the border. To U.S. expansionists, northern Mexico blocked the natural course of U.S. extension westward because it lay athwart a possible railroad route to the Pacific Ocean. Thus, on the eve of the war, Mexico's northern frontier was disintegrating under the pressure of Anglo-American intrusion and the difficulties in managing a frontier so distant from Mexico City.

The remainder of Mexico stretched southward well into the tropics-its southernmost tip about 14° north of the equator. South of the disputed boundary rivers at the Texas border, Mexico was limited on the east by the Gulf of Mexico and on the west by the Pacific Ocean. Thus Mexico was a huge, geographically diverse country essentially bounded by water on its east and west flanks-a condition that made it vulnerable to attack from the sea. Its northern frontier with the expansionist United States occupied a generally arid to semiarid land of relatively low population density and hence was also easy for invading troops to penetrate because they would not face well-defended and well-fortified population centers.

Extending from the tropics to the mid-latitudes and featuring highly extreme topography (e.g., high mountains of 18,700 feet [5,700 meters] near Mexico City and areas below sea level in the California deserts), Mexico was geographically and climatically varied and diverse. The northern and more elevated portions of Mexico in 1846 (e.g., the city of Santa Fe in the province of New Mexico) are characterized by large seasonal temperature variations and extreme winter cold, while the southern and lowland reaches of the country are tropical in climate. Whereas the prevailing tradewinds blow from the Gulf toward tropical Mexico in the vicinity of Veracruz, especially in July when they are most noticeable, the northern part of Mexico reaches well into the mid-latitudes, where westerly winds prevail, as in coastal California. Correlated with both elevation and latitude, precipitation varies from the humid tropical lowlands, which are characterized by wet summers, fairly dry winters, and dense jungle growth, to the mild coast of California, with its characteristically mild, wet winters and dry summers typical of Mediterranean climates. Between the tropics and the mid-latitudes is a huge area of elevated basins and tablelands that are characterized by prolonged drought. Most of this interior upland area has dry winters and hot summers, which are occasionally mitigated by afternoon thundershowers as moisture finds its way inland from either the Gulf of California or the Gulf of Mexico. This huge and climatically and physiographically diverse country was the setting for the U.S.-Mexican War of 1846 and 1847. So widespread were the hostilities in this war, and so distinctive the geography, that virtually every battle was characterized by the geographic environments encountered there by U.S. troops or defended by Mexican forces.

Eastern Coastal Lowlands

The vicinity of Veracruz in southeastern Mexico is characteristic of the Eastern Coastal Lowlands, which are situated on a coastal shelf at the base of mountains and exemplify the tierra caliente (hot country) of the tropical coast, with its high humidity and heavy precipitation, especially in the summer. To the north, the Gulf's subtropical lowlands widen in the vicinity of Corpus Christi, Texas, an area that witnessed the buildup of troops in May 1846 and provided many U.S. soldiers with their first look at the Mexican countryside. There the troops under Gen. Zachary Taylor first praised the invigorating onshore summer breezes, or "tradewinds" as they were erroneously called, but by late fall they cursed the "northers" that brought cold temperatures and rain-weather conditions that debilitated the troops with diseases. The battlefronts along the lower Rio Grande Valley in the vicinity of Resaca de la Palma were adjacent to the coast and had a similar climate. In that area, standing water contributed to its "miasmatic" reputation as mosquitoes bred so readily there. In general, the eastern coastline of Mexico featured a broad coastal shelf and shallow waters less than ten feet deep that made access by sea difficult.

Central Plateaus and Interior Mountains

Troops entering Mexico in the vicinity of Monterrey and Buena Vista ascended into starkly mountainous terrain that was arid for much of the year, with most of the rainfall occurring as summer thunderstorms. This vast area of mountain ranges whose bases are buried in deep alluvial valleys (the drier or more interior of which drain into dry lakes or playas) contained relatively isolated cities, such as Chihuahua and Saltillo. This high semiarid physiographic area was penetrated by U.S. forces on several fronts. For example, Col. Alexander W. Doniphan's troop movements into central Mexico traversed broad basins (or bolsones) punctuated by rugged mountains that trended north to south as part of the basin and range topography of this part of North America. Farther north, the Mormon Battalion traversed similar parts of the basin and range country in southern Arizona, especially in the vicinity of the San Pedro River, where the summer rains are supplemented by an occasional winter rain storm, or snow storm at the higher elevations.

The Mormons began to settle the Great Basin in 1847, when the area was still nominally Mexican territory, and the experience of the Mormon Battalion provided a better understanding of the region's geography and potential f6r settlement, as they hoped to create a theocratic empire called "Deseret." The surveyor and military commander John C. Fremont and others traversed Nevada on their way to California as they crossed the formidable Great Basin. Subsequent exploration revealed that all of these varied locations were physiographically part of the basin and range province, which includes the desert states of present-day Arizona, Nevada, and Utah in the United States and Chihuahua and Sonora in Mexico.

Volcanic Highland Region

Mexico City, the country's largest single population center, is situated in the Volcanic Highland Region, also known as the Sierra Volcanic a Transversal. The soil is rich, the climate mild, and the precipitation dependable, conditions that have helped sustain agriculture and large, densely settled populations there since prehistoric times. During the U.S.-Mexican War this region was the site of important battles such as those at Contreras, Churubusco, EI Molino del Rey, and Chapultepec. Rimmed by tall volcanoes, the valley is situated approximately 7,800 feet (2,600 meters) above sea level. Given its upland subtropical location, the climate is mild despite the altitude and the region is known as tierra templada, or temperate country. When in the Valley of Mexico, U.S. troops found well-watered areas, and the topography made for relatively easy travel. To reach the valley, however, I troops had to ascend the precipitous eastern face of the eastern Sierra Madres. The climate of the higher mountains is perpetually cold in this tierra fria (cold country), and snow covers the tallest peaks all year.

Coastal California

Extending from Alta California into Baja California, the coast was readily accessible to maritime troop movements despite the mountainous country that lay just inland. The numerous harbors provided easy entry for naval forces such as those under the command of Commo. John D. Sloat and Commo. Robert F. Stockton, although the weather conditions (including strong winds and fog) often made for rough going. The coastal portion of California, especially of Alta California, was well populated, having been settled in a series of missions by the Spanish and later the Mexicans. Significantly, most of the population centers that were captured by U.S. forces during the war were close to the sea. Baja California, although less populated, was also easily invaded from the sea because most of the settlements of any size (e.g., Ensenada and La Paz) were coastal ports; the population of the more interior mission communities had dwindled before the war.

Eastern Great Plains

Although not part of the battlefront proper, the plains of extreme northeastern Mexico (present-day eastern Colorado and New Mexico) were important for troop movements. Even by the time of the war they had earned the name "Great American Desert," although in fact the area was covered, at least in part, by prairie grasses. On this front, too, Mexico was vulnerable; invading troops could move deeper into the country from the east traversing the longitudinal river valleys, which provided water, wood, and shelter, as witnessed by the invasion of Santa Fe by troops under Gen. Stephen W. Kearny.


So dominant is the topography and physiography of Mexico that many of the battles in the war gained their logistical character based on the physiographic conditions. For example, the coast in the vicinity of Texas was particularly difficult for maritime operations due to shallow water, and special vessels with shallow draft had to be acquired. In addition, fresh water was so difficult to acquire that on several occasions vessels had to return to Pensacola, Florida, from the Texas and Mexican coasts to replenish water supplies.

Troop movements into the various physiographic aeas also included reconnaissance operations, which helped chart the countryside for future settlement and development that, it was widely believed, would occur after the war. Exemplary in this regard were the explorations of Bryant P. Tilden Jr. on the steamboat Major Brown, which ventured more than two hundred miles up the Rio Grande on an ostensibly military reconnaissance until reaching rapids beyond Laredo. The expedition then continued overland to Presidio del Norte where Tilden noted carefully the prospects for settling the countryside.

At the conclusion of the war in 1848, Mexico lost nearly one-half of its territory to the United States, and the border was formally established along the Rio Grande to El Paso del Norte and thence roughly in a line due west to the Gila River, and then to the west coast. This border in effect cut across the countryside from the coastal subtropical lowlands of Texas to the Pacific coastal lowlands in the vicinity of San Diego, California. Surveying errors and the later realization by the United States that the Gila River did not represent a feasible transcontinental railroad route led to the Gadsden Treaty and Purchase of 1853, which placed the U.S.-Mexican border in its present location, about one hundred miles south of the Gila River. After the war, this arbitrary military boundary became an important zone of contact between the United States and Mexico, the Great Borderlands, which extends along the Rio Grande to El Paso, thence westward along southern New Mexico and Arizona, thence to the coast just south of San Diego, where it divides Alta and Baja California politically. During the war many residents of Baja California had supported the United States, and when the current border was established they were permitted to relocate to Alta California for fear of retribution.

One major geographic consequence of the U.S.-Mexican War was that the United States possessed a well-defined Southwest that reached to the Pacific Ocean as an outcome of the Manifest Destiny sentiments that had been voiced before the war. Mexico, on the other hand, was left with an arid northern frontier ("el Norte"), which remains a developing zone in the 1990s. Two other geographic consequences of the war should be noted. The first relates to the Southern Transcontinental Railroad, which was constructed some thirty years later (1879-1881) as the Southern Pacific Railroad's "Sunset Route" was built from California to New Orleans by way of Yuma, Tucson, El Paso, and San Antonio. Farther north, predecessors of the Santa Fe Railroad were completed across New Mexico and Arizona by 1881 and 1882. The second geographic consequence relates to mining. Despite various reconnaissance missions, Mexico had done little to exploit or develop the mineral resources of its far northern frontier, which was taken by the United States in the war. The discovery of placer gold in California by James Marshall (a Mormon who accompanied the Mormon Battalion) and subsequent placer and hardrock mining ventures by Anglo-Americans revealed that the area Mexico had lost to the United States was a virtual El Dorado of gold, copper, and silver mines.