The Mexican-American War Is the Bloodiest Foreign War the U.S. Has Fought

In the mid-1800s, less than a century after gaining independence from Britain, America's relentless push to expand — prompted, in part, by a belief in Manifest Destiny — was the basis of James K. Polk's presidential run. When he assumed the presidency in 1845, a war with Mexico seemed all but inevitable.

"Not all Americans thought it was a very important war to fight. It was very controversial at the time. But [Polk] wanted to acquire territory," Guardino says. "He was particularly interested in California and New Mexico. New Mexico, at that time, included Arizona and a lot of Nevada, too. He felt like the thing to do was keep marching toward the Pacific and acquiring territory. He really didn't have any other motive."

Still, this was more than a simple land grab. Polk didn't want to simply grab for grabbing's sake. Much of the land he wanted was thought to be useful for agriculture for a growing nation. And California was important for its ports and its animal trade, providing products like leather and tallow (animal fat used to make candles to light homes and factories).

The extra land could be useful politically, too. America at the time was divided over slavery. Any new lands — including those in the Pacific Northwest that Britain controlled — would need to be split to maintain America's tenuous balance between slave and free states.

Mexico, in 1845, was just 24 years removed from its own hard-fought independence, from Spain. David S. Heidler, author (along with his wife, Jeanne) of "The Mexican War," says that the country struggled after independence, but still seemed poised to prosper. "All the smart money would have suspected it might become the dynamic economic and political engine of the Western Hemisphere," Heidler says. "It had so much going for it in terms of its political and cultural and natural benefits."

But its natural resources (mainly, silver) proved harder to cash in than expected, and internal class divisions and political struggles bogged the new country down. Shortly after Polk became U.S. president, he ordered the annexation of Texas — even though Mexico considered the rebellious state its property — and displayed his intention to either purchase other big parts of Mexico or, if the Mexican government wouldn't sell, to take them by force. "I think Polk honestly wanted to write a check and be done with it," Heidler says.

Mexico, then, was trapped into selling land it didn't want to surrender or entering a war that it knew it probably couldn't win. The Mexicans were facing an aggressor who believed it was morally and racially superior, too.

"The U.S. was basically treating the Mexicans the way we had historically treated Native Americans; people who are in the way, and they're not as prosperous as we are, they're racially different, and we had the right to move in there and take what was there," Guardino says. "To many Mexicans, this came as kind of a shock. They didn't think of themselves as being not white, or as being racially inferior. It put Mexican people and politicians in this kind of a bind, where if you don't fight, you're basically saying you're not as good. But if you do fight, you're fighting this terrible war. So they decided to fight."

Polk provoked Mexico by sending troops into disputed territory, a deadly skirmish broke out, and the fight was on. The Mexican-American War cost thousands of lives on both sides.

"In the long run, he got the territory," Guardino says of Polk. "But he paid a much higher cost for it than he imagined."

America's growing economic strength was probably the deciding factor in the war. Mexico was not nearly as powerful, economically or militarily, and lost every major battle in the war.

Still, the Mexicans fought how they could. Though they often were under-equipped, they used guerilla tactics, making this America's first war of that kind on foreign soil. From "The U.S. Army Campaigns of the Mexican War:"

Meanwhile, some of America's non-commissioned Army soldiers (militia, etc.) committed all-out atrocities.

"There were these volunteer regiments with people that really, fully believed that Mexicans were racial inferiors. They treated Mexican civilians extraordinarily poorly," Guardino says. "Lots and lots of theft. Lots and lots of murder. Lots and lots of rape."

The U.S. marched steadily toward Mexico City, entering the capital city and occupying it in a week in September 1847. When Mexican general Antonio López de Santa Anna fled the city — freeing thousands of criminals from prison on his way out — the war was effectively lost, though it would be several more months until a treaty ending the war was signed.

"Once the military was removed from the equation, then it became somewhat of an anarchistic, chaotic mess," Heidler says. "They essentially propped up the government so they could negotiate with it."

The war probably turned earlier, Heidler says, on something thousands of miles away. When the Americans negotiated a treaty with Great Britain for the Oregon Country in 1846, it was clear that the British — who many Mexicans were counting on for help in the war — would not be coming south to help.

By the time the war ended, and Mexico lost its northern territories (including California and New Mexico), some 90,000 Americans had fought and some 14,000 had died. That death rate of 15.5 percent is the highest of any foreign war the U.S. ever has fought, according to the Peace History Society. It's estimated that 25,000 Mexicans died.

"It was a pretty horrific war. By the scale of later wars, it was relatively small," Guardino says. "But ... it was a pretty lethal war."

With the win, Polk fulfilled the dream of Manifest Destiny, the U.S. stretching from the Atlantic to the Pacific. The acquisition of more territory also further kindled America's already inflamed split over slavery, something that erupted 13 years later when Confederate soldiers fired on Union troops at Fort Sumter in what is considered the start of America's Civil War.

The new lands won in the war prompted more westward migration, too, including the California Gold Rush of 1848. Today, the U.S. is considered the richest country in the world, and California and Texas are Nos. 1 and 2 in gross domestic product.

For Mexico, it's been a different story.

"They were simply eviscerated by this and they never really recovered for the rest of the 19th century, some would say never have they recovered," Heidler says. "But to blame the United States solely for that is too convenient.

"Santa Anna remained a power in Mexico after the war, and anything he touched was toxic ... That kind of cronyism, exploitation and corruption within the government was as much responsible for Mexico's troubles as the loss of real estate."

Still, the overt aggression and racism displayed toward Mexico in the war still is felt today, Guardino says, in the way many in the U.S. see Mexicans.

"To most Americans, this is just not very high in their consciousness. They walk around in the landscape, especially in California, where every other name is a Spanish name, and they don't think about how that came to be," Guardino says. "And Americans are very divided on Mexican immigrants. There's a very large number of Americans who really like Mexican immigrants, and there's a very large number of Americans who really don't like Mexican immigrants. But neither one of them connect that like or dislike to what happened in the 1840s."