Texas had been a part of Mexico until it declared itself to be an independent republic in 1836. It existed autonomously until 1846, when the U.S. Congress approved its request to become one of the United States.
While Mexico had tolerated the reality of an independent Texas republic, the Mexican government was not eager to see Texas become part of the Union.
As tensions rose, the likelihood of armed conflict increased, and military planners began to analyze various types of action in Mexico. One officer doing such work was Winfield Scott, who was one of the dominant figures in American military thought in the years between 1815 and 1860: in the years between the War of 1812 and the Civil War.
It was during the War of 1812 that Scott began his career and achieved some fame. He was a senior office by the time tensions with Mexico arose. Brion McClanahan writes:
Before the War Between the States, a handful of American generals had garnered the respect and admiration of the whole American people. Just below George Washington in this pantheon of heroes was Winfield Scott, a man who is hardly remembered today but who was considered by his contemporaries to be without equal. It was not until later, cataclysmic American wars, from the 1860s to the twentieth century, that his fame was eclipsed. Scott was born in Virginia on his family’s plantation, Laurel Branch, in 1786. His father had served during the American War for Independence and his mother counted the prestigious Mason and Winfield families among her ancestors. Scott’s father died when he was six; his mother, when he was seventeen.
One of the points in the tensions between the United States and Mexico was the exact location of Texas’s southern border. Texas claimed the Rio Grande as its southern border, but Mexico argued that the Nueces River was the true border.
President Polk sought a diplomatic solution with Mexico. He sent representatives to offer a deal to the Mexican government: a large sum of cash for the disputed regions of Texas, for New Mexico, and for California.
The attempted negotiations did not go well. The Mexican government was not inclined to yield any land, and the leadership of the government was in turmoil: key posts changed occupants several times within a year, so that diplomats from the United States were often negotiating with someone who’d just obtained the position and who would soon leave it.
Polk had sent General Zachary Taylor, first to Louisiana, and then into Texas, aware that military action might be in the offing. Taylor’s presence was primarily defensive, yet perhaps provocative.
When the seemingly inevitable war finally began, Taylor’s forces would operate along the border and into the northernmost regions of Mexico. Winfield Scott, however, would take the fight home to the Mexican heartland.
In addition to Taylor and Scott, other officers had significant roles. While Taylor, in conjunction with Brigadier General John Wool, worked from eastern Texas into northern Mexico, Colonel Kearny and Colonel Doniphan started from western Texas, Kearny heading west for southern California, and Doniphan heading south toward the Mexican town of Chihuahua and other points further south.
Wool, Doniphan, and Taylor would campaign around northeastern Mexico, in the vicinities of Monterrey and Saltillo.
President Polk was hoping for a short war, but General Winfield Scott had foreseen the likelihood of a longer engagement. Polk had given an important role to Taylor, partly for political reasons, at the beginning of the war. Taylor and Scott both had political potential and ambition. Taylor would be nominated by the Whig party for the presidency in 1848, Scott in 1852: Taylor successfully, Scott not.
Foreseeing such political activity, Polk hoped to give Taylor a strong public image by placing him in charge of the northern military operations against Mexico. Polk’s plan largely succeeded, and Taylor was viewed as a strong and successful military leader, boosting his political potential.
But the northern aspect of the Mexican war proved to be insufficient. Scott understood that engagement further south in Mexico would be necessary for victory. As the war continued, Polk eventually came to see the merit of Scott’s line of thought.
In Washington in November 1846, Polk and Scott decided to proceed with invasion plans. An amphibious landing would be made on Mexico’s eastern coast, at the town of Vera Cruz. Mapmakers sometimes make one word of the town's name: Veracruz. From there, Scott would lead the forces west toward Mexico City.
Scott appropriated significant numbers of soldiers from Taylor’s forces, which angered Taylor. Scott’s forces landed and captured the Vera Cruz in March 1847. Historian Russell Frank Weigley writes:
The object of Scott’s campaign was far clearer: to convince the Mexican government of the futility of prolonging the war, and perhaps to make its continued prolongation impossible, by capturing the capital and heart of the country, the City of Mexico. The territorial gains which the United States had sought in going to war against Mexico were already in the American hands or about to be when Scott’s campaign began. Taylor had assured American possession of Texas; and the combined efforts of the American settlers in California, the American Navy’s eastern Pacific squadron, John C. Fremont’s opera bouffe, and Brigadier General Stephen Watts Kearny’s Army of the West were capturing California. Kearny dropped off enough garrisons between Texas and California to terminate Mexican sovereignty in the intervening territory, especially when shielded by the anabasis of Colonel Alexander Doniphan’s one thousand across Chihuahua. Scott’s task was to persuade a Mexican government to grant formal recognition to these developments so that the United States could go about exploiting its conquests in as much peace as the resident Indians would permit.
Although Polk had wanted to fuel Taylor’s political chances, he saw that Scott was the wiser strategist. Taylor was worried that his electoral opportunities were being damaged. His worries, in hindsight, were misplaced. Taylor would end up in the White House; Scott would not.
Taylor’s nickname was “Old Rough and Ready,” which played well in the press. Scott’s nickname was “Old Fuss and Feathers,” because he insisted on discipline and uniforms. Taylor’s nickname was another political advantage. Historians Allan Millett and Peter Maslowski write:
Scott was an excellent choice. Since the war began he had argued that only a repetition of Cortes’s march to the valley of Mexico would end the war. When the administration first contemplated the expedition, Scott wrote the planning papers detailing the military requirements and establishing the operation’s feasibility. He estimated that 4,000 regulars and 10,000 volunteers would be needed and insisted that the Veracruz assault had to take place before yellow fever season began. Since little time remained to raise new regiments, Scott took more than half Taylor’s men, including almost all his regulars, and prudently ordered Old Rough and Ready to remain on the defensive. The expedition was a double blow to Taylor. Denied the opportunity to command it, he also lost most of his army. Polk and Scott, he fumed, had conspired to cut short his military career and deprive him of the 1848 Whig nomination.
The countryside which Scott entered was not unified. The internal dynamics of Mexican politics were determined by demographics. The turmoil at the top levels of the Mexican government - the rapid turnover of even the highest offices - mirrored deep divisions within Mexican society.
Some of these divisions had been fostered by the Spaniards earlier in Mexican history. Although Mexico had been independent of Spain since the early 1820s, various segments of society remained at odds with each other.
This would manifest itself during the war, as U.S. forces were welcomed in some areas and hated in others, and after the war, when the United States quickly became an ally of the nation it had so recently defeated. Historian Irving Levinson writes:
Unlike its British North American counterparts, colonial Mexico was a land in which the Indians and their offspring permanently outnumbered the European settlers and their descendants. Almost three centuries after European settlement began, the criollos (native-born whites) and the resident Spaniards together comprised barely 20 percent of Mexico’s population. That minority offered limited opportunity to the remainder of the population as the political, economic, and social levers of power in New Spain remained primarily under the control of Spanish officials and their acquisitive, resentful criollo colonists. In this environment, an arrangement emerged in which Indians who sought to retain control of some of their remaining preconquest lands received the protection of the Spanish Crown. In turn, the royal government used the Indians as a counterweight to colonists who sought to expand their own political and economic power at the expense of the metropolitan regime in Madrid. Also, the colony remained culturally divided as differing concepts of land ownership and political power generated continual conflict. These factors, combined with the heritage of a cruel Spanish conquest and of horrific conditions of forced labor, prevented the emergence of a unified colonial society.
While the Mexican government was not happy to lose the contested territory between the Rio Grande and the Nueces River, a significant number of Mexicans embraced the idea that the United States was the promulgator of the Monroe Doctrine.
The Mexicans had been encouraged by the example of the United States when they sought and gained their independence from Spain. The United States would also assist the Mexicans in retaining their independence when France attacked and attempted to subjugate Spain in the 1860s.