Mexican-American War, Part I

President James Polk, circa 1846. | Courtesy Library of Congress

Historians often refer to the Mexican-American War (1846-1848) as the forgotten war since it was quickly overshadowed by the discovery of gold in California, which occurred just as the peace treaty was signed and the United States took possession of the former Mexican province. The conflict was then further eclipsed by the cataclysmic American Civil War 13 years later. But it was war with Mexico that exacerbated deep sectional divisions in the United States triggered by slavery and the potential expansion of America’s “Peculiar Institution” to new Western territories that led directly to that Civil War.

The Mexican-American War was supported by most Americans during the first year of impressive U.S. battlefield victories, but it later proved highly divisive and harshly judged as it wore on.

In 1844, James K. Polk, a slave-owning Democrat from Tennessee was narrowly elected president of the United States over his opponent, noted statesman and legendary orator Henry Clay. Polk campaigned on a platform of land acquisition in the West including the controversial annexation of Texas, a goal that led to the hot-wire topic of adding more slave territory to the nation. Clay was a slave owner, too; however, as a politician who had earned the appellation the Great Compromiser for his ability to defuse sectional crises, he felt expansion of slavery would upset the delicate political balance of the country. Clay’s position did not fit the surging narrative of Manifest Destiny, which cost him a close election. Although he would not live to see the Civil War, Clay was correct about the issue of slavery expansion leading to national instability and greater divisional conflict in the United States.

Read Part II, Part III & Part IV

True to his word, in December 1845 President Polk approved the annexation of the 10-year-old Republic of Texas, a slave-holding territory. He brought Texas into the fold, but kept his eyes focused on the West. Polk, along with members of his cabinet, rightfully feared rival Great Britain had its own designs on California and the Pacific Coast. An 1818 treaty had created joint control between the U.S. and England for the Oregon Country, a vast territory west of the Rocky Mountains and north of the 49th parallel. President Polk was a staunch supporter of the growing movement of Manifest Destiny in the 1840s, where native-born, white Americans “possessed the divine right” to expand west to the Pacific Ocean.

In Polk’s mind, and in reality, Great Britain’s presence in the Pacific Northwest and Mexico, with its control of California, complicated the American push west. The president secretly sent diplomats to England to wrest control of the Oregon Country from the world’s greatest military power at the time. In a brazen move that was half bluff, Polk threatened war to gain the territory. It was a bold maneuver and Polk was genuinely surprised when Britain blinked and signed a treaty relinquishing the future states of Oregon, Washington, Idaho and western Montana to the U.S. It was a brilliantly played acquisition. Buoyed by the results of this successful strategy, Polk decided to try the same ploy with Mexico, this time offering money instead of impending war. At first it looked promising; Mexico’s President Joaquin de Herrera indicated openness to the deal, but he was overthrown in January 1846 by a cabal of conservative Army generals unwilling to negotiate with the United States.

While all this geo-political intrigue was in motion, the long-sought California Trail over Donner Pass was established. In April 1846, members of what would become the Donner Party joined about 1,500 other pioneers with covered wagons heading west to Mexico’s Alta California, to settle there and establish American communities and institutions. They were predominantly agrarian families looking to put down new roots. The same month those emigrant wagons rolled west from the Missouri River, hostilities broke out between Mexican and American military forces over a festering border dispute along the Rio Grande River.

The Mexican government never officially recognized the Republic of Texas or its southern border and stated (correctly) that the legal border was the Nueces River about 100 miles northeast of the Rio Grande. On April 25, Mexican troops crossed the Rio Grande to attack U.S. forces that Polk had provocatively positioned on the north side of the river. Predictably, in response to Mexico drawing first blood, Polk declared that Mexico’s defense of its sovereign territory an invasion of American soil and on May 13 Congress declared war.

The conflict had no direct impact on the California Trail in 1846, but later during the winter of 1846-47, when it came time to rescue the ill-fated Donner Party trapped by deep snow in the Sierra, relief efforts were stymied because many recently arrived American settlers were away fighting in Mexico.

Perhaps no war should be forgotten because there are important lessons to be learned from military conflict, as well as soul searching to determine if the war served the people’s and nation’s best interest. The war was vigorously supported by most Americans during the first year of impressive U.S. battlefield victories against overwhelming odds, but it later proved highly divisive and harshly judged as it wore on.

In his memoirs, Ulysses S. Grant, a U.S. president and commanding Union general in the Civil War, called the armed conflict “a wicked war.” The West Point graduate admitted that he had held that opinion even as he bravely led troops in battle against Mexico, but “I had not moral courage enough to resign.” Grant wrote that the American-instigated conflict was “one of the most unjust wars ever waged by a stronger against a weaker nation, an instance of a republic following the bad examples of European monarchies.” He declared that “the American Civil War was punishment for that transgression.”

Many disagreed with Grant’s assessment, but this war — the first ever fought by American troops on foreign soil — produced cautionary military metrics that still resonate. U.S. forces were plagued by more than 11,000 desertions, the highest rate of troop desertion of any American war at more than 8 percent. The conflict also caused the highest death rate of any war, including the horrific Civil War. According the U.S. Dept. of Defense, 13,200 of the 79,000 men who served in the Mexican war died, a mortality rate of nearly 17 percent, compared to 6.5 percent in the Civil War. Most American soldiers perished from disease, but more than 1,000 men were reported missing, likely due to irregular Mexican forces who picked off American troops one-by-one in guerrilla attacks during the subsequent occupation. Mexican casualties among soldiers and civilians have been estimated at about 25,000.

Stay tuned for Part II in the next edition and at