What started the Mexican-American War

On May 13, 1846, the United States declared war on Mexico, beginning the Mexican-American War. The Mexican-American War is one of the least known pivotal moments in US History. It paved the way for so many other important events, from the expansion and dispossession of indigenous people, the California Gold Rush, and American Civil War. It added the states of California, Texas, New Mexico, Utah, Arizona, Nevada, and parts of Colorado and Wyoming to the United States. American success in the war solidified belief in the concept of Manifest Destiny, confirming the idea that the United States had been destined by divine entities to expand into a continental empire resembling the present-day nation. For proponents of slavery and abolition, the new territory provided a source of conflict over whether slavery would expand and continue in the West. For the major military leaders of the Civil War on both the Confederate and Union sides, including such Pig War figures as Winfield Scott, George Pickett, and William Harney, it was a crucible in which their skills were tested, their tactics were formed, and they built the bonds of camaraderie that they relied on during the Civil War. The immediate cause of the Mexican-American War was a disputed boundary between the United States and Texas on the Nueces Strip. Mexico did not recognize Texas as legitimate American territory and Texas admission to the United States antagonized Mexican officials and citizens. Rather than ameliorate this problem, President Polk intentionally worked to aggravate Mexico and provoke a war. On January 13, 1846, Polk ordered American forces into deeply disputed territory. In April, an army of approximately 4,000 men lead by General Zachary Taylor entered the Nueces Strip, a contested territory that Mexico and many Americans regarded as never having been a part of Texas. Polk knew this action would antagonize Mexican military forces stationed within sight of Taylor’s army at Matamoros. Colonel Hitchcock, who served with Zachary Taylor’s army, could see the real intention of his deployment from his vantage point on the front lines: “We have not one particle of right to be here. It looks as if the government sent a small force on purpose to bring on a war, so as to have a pretext for taking California and as much of this country as it chooses.” 

On April 9, Taylor’s army deployed the bulk of its artillery within striking distance of the 16,000 person city of Matamoros, Mexico. The US Navy then cut Matamoros off from the Gulf of Mexico, creating a blockade by land and water of the Mexican civilians and soldiers just across the Nueces River from Taylor’s forces. On April 24, Mexican forces crossed the Matamoros River to break the blockade; Taylor sent a small force to respond, and the Mexican Army opened fire, killing 11 American soldiers. The site of this battlefield is now preserved as Palo Alto Battlefield National Historical Park.

The US military was wildly successful and used a combination of innovative strategy, competent officership, luck, and brutality to achieve victory. American forces captured Mexico City within 16 months of the war’s beginning and expeditionary forces also fought battles in California and New Mexico that enabled the seizure of vast tracts of territory. As the Mexican-American War came to an end, gold was discovered in California, causing a massive population surge on the West Coast that ensured continued American sovereignty over the entire West Coast and paved the way for the boundary conflict in the San Juan Islands.

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The Mexican-American War of 1846-1848 was a combination of Mexican unwillingness to recognize Texas independence, the desire of Texans for statehood, and American desire for westward expansion.

Tensions between Mexico and the United States had been building ever since the Mexican government invited Anglo settlers into Texas in the 1820s.

When Texas formally declared independence and defeated Santa Ana at the Battle of San Jacinto, Santa Ana did not specifically recognize Texan independence.

The United States government claimed the Mexican authorities were abusing American vessels and citizens at the ports. Mexico failed to pay the United States for the claims.

The United States wished to expand to the Pacific Ocean. The term “Manifest Destiny” described how most Americans felt about the rights of their nation to acquire more territory.

The Mexican government in the years leading up to the war was highly unstable, which kept Mexico from maintaining a consistent foreign policy. Between 1821 and 1846, the Mexican government changed hands eleven times, usually as a result of a palace revolution.

The Mexicans maintained an ongoing border disagreement with Texas from 1836 until the outbreak of the war. Texas claimed the Rio Grande River as its southern boundary while Mexico claimed the Nueces River that was almost 100 miles further north as the boundary.

The United States’ quick offer of annexation to Texas enraged the Mexican government and seemingly left them no choice but to follow through on their threats of war.

America and Texas were determined that Texas should join the union, and Mexico was equally determined to prevent that from happening. Mexico might have recognized Texas independence and eventually even the annexation by the United States. What brought on war was the quickness of American action combined with the Mexican efforts to prevent the annexation of Texas by the United States.

Source: The Causes and Reasons for the Mexican-American War
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Ulysses S. Grant | Article

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In 1844, President James K. Polk ran on a Democratic platform that supported manifest destiny, the idea that Americans were predestined to occupy the entire North American continent. The last act of Polk's predecessor, John Tyler, had been to annex the Republic of Texas in 1845. Polk wanted to lay claim to California, New Mexico, and land near the disputed southern border of Texas. Mexico, however, was not so eager to let go of these territories.

Polk started out by trying to buy the land. He sent an American diplomat, John Slidell, to Mexico City to offer $30 million for it. But the Mexican government refused to even meet with Slidell. Polk grew frustrated. Determined to acquire the land, he sent American troops to Texas in January of 1846 to provoke the Mexicans into war.

When the Mexicans fired on American troops in April 25, 1846, Polk had the excuse he needed. He declared, "[Mexico] has invaded our territory and shed American blood upon American soil," and sent the order for war to Congress on May 11.

The act was a questionable one. Many Northerners believed that Polk, a Southerner, was trying to gain land for the slaveholding South. Other Americans simply thought it was wrong to use war to take land from Mexico. Among those was Second Lieutenant Ulysses S. Grant. Although during the war he expressed no reservations about it, he would later call the war "one of the most unjust ever waged by a stronger against a weaker nation. It was an instance of a republic following the bad example of European monarchies, in not considering justice in their desire to acquire additional territory."

Despite arguments over whether the war was right, Americans had tremendous success on the battlefield. Young officers like Grant and Robert E. Lee, who would later lead armies against one another in the Civil War, had their first combat experiences in Mexico. Generals Zachary Taylor and Winfield Scott won a series of remarkable victories against the Mexican armies. This success was in spite of the fact that Mexican troops outnumbered the Americans in most cases. In September of 1847, after a masterful overland campaign, American troops under Scott captured Mexico's capital, Mexico City, and the fighting ended.

The Treaty of Guadalupe Hidalgo sealed the American victory in 1848. In return for $15 million and the assumption of Mexican debts to Americans, Mexico gave up its hold over New Mexico and California. The enormous territory included present-day California, Nevada, Utah, and parts of Arizona, New Mexico, Oklahoma, Colorado and Wyoming. Mexico also agreed to finally relinquish all of Texas, including the disputed area along the border. The U.S. Congress approved the treaty on March 10.

Although the Mexican War had been won, the conflict over what to do with the vast amounts of territory gained from the war sparked further controversy in the U.S. The question over whether slavery would spread to these new territories would drive North and South even further apart.