America’s Desire to Acquire California

Americans believed that spreading American democracy and freedom across the North American continent was God’s will and their historical mission. As the famous poet Walt Whitman stated, the expansion of American power and territory served the interests of humanity—the larger, the better.

In reality, these lands were far from Mexico’s political center and sparsely populated. Mexico’s control over them was extremely weak, with no permanent administrative institutions. The land was essentially open and unmanaged.

Since the westward movement began in 1805, tens of thousands of American settlers had already moved into these regions over several decades. In the frontier era, actual occupation depended on population presence; without settlers, claims were merely nominal. By 1840, California had a population of over 90,000, more than 90 percent of whom were Americans.

The United States proposed purchasing California from Mexico. The American offer included $5 million for New Mexico and $25 million for California. If both proposals were rejected, the U.S. was prepared to offer $5 million for part of California.

In December 1845, American envoy John Slidell arrived in Mexico City to negotiate. The Mexican government refused to meet with him. Slidell had no choice but to return home, firmly convinced that the United States could acquire these Mexican territories only by force.

President James K. Polk (1795–1849) agreed with Slidell’s assessment. Polk had originally hoped for a peaceful resolution, but that hope appeared increasingly unrealistic.

Causes of the War

The Mexican–American War originated from disputes over the Republic of Texas. The Republic was much larger than today’s state of Texas. The region had originally been under Spanish rule. Spain had maintained control for over 300 years, but the area remained sparsely populated and undeveloped, as Spain focused its attention on wealthier Central and South America.

After Mexico gained independence in 1821, this vast territory became part of Mexico. During the westward expansion of the 1820s, large numbers of American settlers migrated into Texas. By the 1830s, the population had grown to tens of thousands.

In 1836, local settlers established the Republic of Texas and declared independence from Mexico. Mexico refused to recognize the new republic, and the borders it claimed were disputed. Shortly thereafter, a Mexican dictator personally led troops northward, attacking Texas forces. Within two weeks, about 200 Texan defenders were wiped out.

However, one month later, General Sam Houston led 800 Texan troops to defeat Mexican forces, killing or capturing 1,600 soldiers, including the Mexican general himself. Only nine Texans were killed. This decisive victory secured Texas’s de facto independence.

Outbreak of the War

In 1845, the Republic of Texas joined the United States as its 28th state. Mexico refused to recognize this decision and continued to dispute the border, declaring its intention to reclaim Texas.

In April 1846, Mexican cavalry attacked and captured American soldiers. In May, the United States formally declared war on Mexico.

In 1847, the Mexican president personally led troops northward but suffered defeat. American forces entered California, while the U.S. Navy launched amphibious operations against Mexico’s central regions, eventually capturing Mexico City.

In February 1848, both sides signed an armistice. The United States deployed about 60,000 troops, while Mexico fielded approximately 40,000. American casualties exceeded 10,000, while Mexican losses surpassed 20,000.

Results of the War

Under the armistice terms, northern California, Nevada, all of Utah, and parts of Colorado, Arizona, New Mexico, and Wyoming were ceded to the United States. Mexico lost half of its territory—approximately 2.3 million square kilometers. The United States paid $18.25 million as compensation.

After the war, Mexico was forced to accept the new reality. Thousands of Mexicans already living in these regions either returned to Mexico or remained in the United States, becoming the earliest Mexican Americans.

Impact on the United States and Mexico

The Mexican–American War fostered a sense of nationalism in Mexico that had been lacking since its founding. It contributed to the establishment of the liberal republic in 1857, marking a departure from long-standing authoritarian rule. The new republic accelerated settlement of the sparsely populated northern regions to prevent future territorial losses.

For the United States, the war connected the Atlantic and Pacific coasts into a single continental entity, accelerating the overall development of North America. In 1848, Friedrich Engels commented that America’s annexation of California and acquisition of Pacific dominance served the interests of the entire continent’s development.

Marx and Engels further noted that the significance of the acquisition lay in California’s rich resources, which stimulated the world market, bringing prosperity to both the American West Coast and East Asia, and creating new global markets.

At the time, Mexico lagged behind the United States—and continues to do so today. Had the massive territorial changes of 1848 not occurred, the western regions of North America might today resemble underdeveloped areas of Mexico.

Currently, more than 400,000 undocumented migrants cross from Mexico into the United States annually. Over time, the U.S. has accumulated an estimated 30 million undocumented immigrants awaiting resolution, becoming a persistent domestic challenge.

Polk’s Early Death

President Polk retired after a single four-year term, during which U.S. territory expanded by one-third—an achievement of historic significance. He declined reelection.

Overworked during the Mexican–American War, Polk routinely worked 15-hour days, damaging his health. He died just over three months after leaving office, at the age of 54.