The Fall of Mexico City:

On September 13, 1847, the Americans, under the command of General Winfield Scott, took the fortress at Chapultepec and the gates to Mexico City: they were close enough to fire mortar rounds into the heart of the city. The Mexican army under General Antonio Lopez de Santa Anna abandoned the city: he would later try (unsuccessfully) to cut the American supply lines to the east near Puebla. The Americans took control of the city. Mexican politicians, who had previously stalled or rebuffed all American attempts at diplomacy, were ready to talk.

Nicholas Trist, Diplomat

Some months before, American President James K. Polk had sent diplomat Nicholas Trist to join General Scott's force, giving him authority to conclude a peace agreement when the time was right and informing him of the American demands: a huge chunk of Mexico's northwestern territory. Trist repeatedly tried to engage the Mexicans during 1847, but it was difficult: the Mexicans did not want to give away any land and in the chaos of Mexican politics, governments seemed to come and go weekly. During the Mexican-American War, six men would be President of Mexico: the presidency would change hands between them nine times.

Trist Stays in Mexico

Polk, disappointed in Trist, recalled him in late 1847. Trist got his orders to return to the USA in November, just as Mexican diplomats began seriously negotiating with the Americans. He was ready to go home when some fellow diplomats, including Mexican and British ones, convinced him that to leave would be a mistake: the fragile peace might not last the several weeks it would take a replacement to arrive. Trist decided to stay and met with Mexican diplomats to hammer out a treaty. They signed the pact in the Guadalupe Basilica in the town of Hidalgo, named for Mexico's founder Father Miguel Hidalgo y Costilla, and which would give the treaty its name.

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Treaty of Guadalupe Hidalgo

The Treaty of Guadalupe Hidalgo (the full text of which can be found in the links below) was nearly exactly what President Polk had asked for. Mexico ceded all of California, Nevada, and Utah and parts of Arizona, New Mexico, Wyoming and Colorado to the USA in exchange for $15 million dollars and forgiveness of about $3 million more in previous debt. The treaty established the Rio Grande as the border of Texas: this had been a sticky subject in previous negotiations. Mexicans and Indigenous communities living in those lands were guaranteed to keep their rights, properties, and possessions and could become US citizens after one year if they desired. Also, future conflicts between the two nations would be settled by arbitration, not war. It was approved by Trist and his Mexican counterparts on February 2, 1848.

Approval of the Treaty

President Polk was enraged by the refusal of Trist to abandon his duty: Nevertheless, he was pleased with the treaty, which gave him all that he had asked for. He passed it along to Congress, where it was held up by two things. Some northern Congressmen tried to add the "Wilmot Proviso" which would assure that the new territories did not allow enslavement: this demand was later taken out. Other Congressmen wanted even more territory ceded in the agreement (some demanded all of Mexico!). Eventually, these Congressmen were outvoted and Congress approved the treaty (with a couple of minor changes) on March 10, 1848. The Mexican government followed suit on May 30 and the war was officially over.

Implications of the Treaty of Guadalupe Hidalgo

The Treaty of Guadalupe Hidalgo was a bonanza for the United States. Not since the Louisiana Purchase had so much new territory been added to the USA. It would not be long before thousands of settlers began making their way to the new lands. To make things even sweeter, gold was discovered in California shortly thereafter: the new land would pay for itself almost immediately. Sadly, those articles of the treaty which guaranteed the rights of Mexicans and Indigenous communities living in the ceded lands were often ignored by Americans moving west: many of them lost their lands and rights and some weren't officially given citizenship until decades later.

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For Mexico, it was a different matter. The Treaty of Guadalupe Hidalgo is a national embarrassment: the lowlight of a chaotic time when generals, politicians and other leaders put their own self-interests above those of the nation. Most Mexicans know all about the treaty and some are still angry about it. As far as they're concerned, the USA stole those lands and the treaty just made it official. Between the loss of Texas and the Treaty of Guadalupe Hidalgo, Mexico had lost 55 percent of its land in twelve years.

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Mexicans are right to be indignant about the treaty, but in reality, the Mexican officials at the time had little choice. In the USA, there was a small but vocal group that wanted much more territory than the treaty called for (mostly sections of northern Mexico that had been captured by General Zachary Taylor during the early part of the war: some Americans felt that by "right of conquest" those lands should be included). There were some, including several Congressmen, who wanted all of Mexico! These movements were well known in Mexico. Surely some Mexican officials who signed off on the treaty felt that they were in danger of losing much more by failing to agree to it.

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The Americans weren't Mexico's only problem. Peasant groups all over the nation had taken advantage of the strife and mayhem to mount major armed revolts and insurrections. The so-called Caste War of Yucatan would claim the lives of 200,000 people in 1848: the people of Yucatan were so desperate that they begged the US to intervene, offering to willingly join the USA if they occupied the region and ended the violence (the US declined). Smaller revolts had broken out in several other Mexican states. Mexico needed to get the US out and turn its attention to this domestic strife.

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In addition, the western lands in question, such as California, New Mexico, and Utah, were already in American hands: they had been invaded and taken early in the war and there was a small but significant American armed force already in place there. Given that those territories were already lost, was it not better to at least gain some sort of financial reimbursement for them? Military reconquest was out of the question: Mexico had been unable to re-take Texas in ten years, and the Mexican Army was in tatters after the disastrous war. The Mexican diplomats probably got the best deal available under the circumstances.

Sources

The Editors of Encyclopaedia Britannica Edit History

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https://www.thoughtco.com/the-treaty-of-guadalupe-hidalgo-2136197

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Eisenhower, John S. D. "So Far From God: The U. S. War With Mexico, 1846–1848." Paperback, University of Oklahoma Press, September 15, 2000.

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Henderson, Timothy J. "A Glorious Defeat: Mexico and Its War with the United States." 1st edition, Hill and Wang, May 13, 2008.

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Wheelan, Joseph. "Invading Mexico: America's Continental Dream and the Mexican War, 1846-1848." Hardcover, 1st Carroll & Graf Ed edition, Carroll & Graf, February 15, 2007.