The Real Reason The Mexican-American War Happened

In his influential 1849 essay "Resistance to Civil Government," what has gone down in history as "Civil Disobedience," the Transcendentalist writer Henry David Thoreau wrote the famous line: "That government is best which governs least." And as noted by the Constitutional Rights Foundation, that attitude was inspired by, among other acts of the U.S. government with which he disagreed, the Mexican-American War. Thoreau was actually jailed for refusing to pay his taxes and speaking out against the country's move to invade and occupy Mexico. It was his conviction that he and other people of conscience had a duty to actively refuse to support institutions like slavery and an unjust war that he saw as indefensible land-grabbing.

Fought from 1846-1848, the war resulted in Mexico's loss of around one third of its territory. According to, the United States gained the present-day states of New Mexico, Arizona, Utah, Nevada, and California after the Treaty of Guadalupe brought the bloody struggle to an end in February 1848. But why was the war waged in the first place? Let's take a look at the causes of the Mexican-American War and see what reasons the United States had for starting such a conflict with its neighbor to the south.

The United States was 'destined' to fight the Mexican-American War

According to Britannica, by the time the Texas was annexed to the United States in 1845, many U.S. citizens were convinced that the country was "destined" to extend its borders as far as the Pacific Ocean — or as far as possible, really. This classically American attitude was known as Manifest Destiny. It provided a divine pretext for things like genocide, slavery, land appropriation, invasion, and just about any other unscrupulous act that may have resulted in more land and resources for the United States. As evinced by people like Thoreau, the entire country wasn't behind the decision to go to war with Mexico, but enough of the citizenry had an attitude of Manifest Destiny for the war to happen.

While the United States considered the Rio Grande River to be the southern border of Texas, Mexico said it was the Nueces River to the north, so the area between the two became disputed territory. After the Battle of Resaca de la Palma at Palo Alto, Texas, on May 8, 1846, President James K. Polk told Congress that the Mexican army had "invaded our territory and shed American blood on American soil." And that was enough for the Congress to declare war on Mexico.