This post on the Mexican-American War Causes is a continuation of my previous post. Part one (read here) detailed the reasons why the US and Mexico were upset with each other in 1845. Today’s blog describes the events that caused war to break out in 1846, but it won’t make full sense unless you’ve read part 1.

Mexican-American War Causes – The Disputed Region

Luckily for those Americans who craved a war to extend slavery, a dispute existed over the western border of Texas. Mexico claimed it was the Nueces River.  The US claimed it was the Rio Grande River. The dispute meant about 40% of modern Texas hung in the balance.

The US sent a commissioner to settle things. But the commissioner, John Slidell, also had orders to buy California and New Mexico for $30 million. Understandably enough, the Mexicans turned Slidell away. They didn’t feel like selling their country.

James Polk, a slaveholder from Tennessee, was the US president by now, replacing John Tyler. He ordered General Zachary Taylor into the disputed area.

Mexican-American War Causes – The Disputed History

This is where things get muddy. Historians are in general agreement about everything I’ve written so far in this post and in part 1. The next part is where disagreement exists. Taylor skirmished with some Mexican forces. A few people died.

President Polk claimed Mexico had invaded the US and asked for a declaration of war from Congress. He got it. But not without opposition, especially from the Whig Party. (Polk was a Democrat.)

One famous example of the opposition came from a freshman representative from Illinois, Abraham Lincoln. He made his famous “spot resolutions,” a request for Polk to show the exact spot American blood had been shed on American soil. Former president John Quincy Adams also disagreed with the war.

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Even some of the soldiers in Mexico thought the whole war was a sham. A group of Irish soldiers deserted to the Mexican side and became the San Patricio Battalion.

Understandably, historians have been divided about Polk’s moves ever since. Was the whole thing a charade? Were Americans really killed on American soil? Or were Polk’s decisions just clever diplomacy to get Congress to declare a war America was likely to win? Agreement remains elusive on these questions.

The Consequences of American Victory in the War

Sadly, I’m not able to untie the knot described above. But 1840s Americans drew their own conclusions. Many of the volunteers who fought in the war were Southerners because people in the North didn’t want to fight Mexico simply to extend slavery. (Recall this is when Henry David Thoreau went to prison for not paying taxes, believing those taxes went to fight an unjust war.)

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The belief in a “Slave Power Conspiracy” to control the US government grew among Northerners. Some Northern politicians became more outspoken in their opposition to slavery. This helped fuel the growth of abolitionism as a force in the US.

Southerners were happy, though. Texas was now American, as was the present-day US Southwest and California, thanks to the Mexican War. Slavery had much more potential territory into which it could expand.

Extended Consequences

Many historians date the causes of the Civil War to the annexation of Texas and the Mexican-American War. I’m among them. This was the first major event that caused an important political split between North and South that future events were able unable to mend.

Many other things happened in the years to come, of course. Civil war was not inevitable after the end of the Mexican-America War in 1848. But this was the first big step in that direction.

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