Causes of the Mexican-American War

The Mexican-American War occurred between 1846 and 1848 and was a war that took around 30,000 lives during the course of the conflict. The question is, with this being such a large scale war causing so many deaths why don’t you know about it?

This is a great question and the answer generally comes to be that the most important and well known war of the century from a United States perspective is the American Civil War.

The people of the United States will know of the Mexican-American War, but even they may not know the causes so today we will talk about the causes of the Mexican-American War.

Causes of the Mexican-American War


Between 1835 and 1836 the Texas Revolution occurred. This revolution was because Texas, being a part of Mexico at this time, was not happy about the Mexican government. One of the biggest bones of contention was economic because the Mexican government micromanaged peoples land by telling them what they could and couldn’t grow.

With the boom of the cotton industry and the large revenues it generated many land owners wished to start a cotton plantation, the Mexican government only wanted them to grow corn and other foodstuff or have cattle.

Another issue was that many of the settlers in Texas were American migrants and they wanted to use slave labour as a form of managing their land at a low cost, the Mexican government was against slavery too.

The outcome of the Texas Revolution was that Texas won and gained independence from Mexico at the Treaties of Velasco.

At this point Mexico had promised that should Texas succeed in then being annexed into the United States they would wage war against the U.S.

For ten years Texas ran as an independent state, Mexico however still deemed it a Mexican state that was being rebellious and that would eventually fall back under the umbrella of the Mexican government. The United States, Britain and France however had all recognised Texas as its own state and advised Mexico not to try and retake the lands with force.

In 1845 things changed when the Americans reviewed whether to tender to Texas the idea of becoming a part of the United States. Once the tender was put forward Texas readily agreed and the U.S. Congress made Texas a state of America in December 1845. This angered the Mexicans

As part of the annexation of Texas into the United States the dispute on Texas boundaries caused more anger as American President Polk claimed the Rio Grande River as part of Texas and the border between Mexico and Texas.


With all this going on there was also the issue of another Mexican territory, California. The United States for ten years, ever since Texas independence, had a roving eye on California and they wished to annex these lands too. Initially the American stance was to enter negotiations with Mexico to be given the lands in lieu of debts.

Mexico didn’t agree to the move so an intrepid American explorer called John Fremont took a small band of armed men and entered California on the pretext he was buying supplies. Once in California he then states he wanted to buy a home near the sea for his mother.

The Mexicans didn’t believe the story of Fremont and asked him to leave California but Fremont decided a different course of action and built a fort on the mountainous Gavilan Peak (now Fremont Peak). Here Fremont raised the American flag before leaving California for a small time and then returning. Upon his return in 1845 he mustered the American immigrants into a revolt declaring the independence of California from Mexico.


The Americans sent John Slidell to Mexico City to try and negotiate but the Mexicans were not interested and carried on their view of Texas being a part of Mexico. Slidell returned to the United States and Mexico started building an army.

This army went north and in 1846 2,000 Mexicans attacked a 70 strong U.S patrol killing 16 and thus giving America reason for war.

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