In 1848, the US ended a war with its neighbour and took its land. What happened next?

In February 1848, after almost two years of conflict, Mexico signed a peace treaty that formally ended a war with the United States which it no longer had any chance of winning. But the peace treaty didn’t only mark an end to the war. It saw the formal loss of a huge swathe of Mexican territory to the victorious US, significantly expanding the American frontier to the west and south and marking a new border.

This was only one more chapter in the long history of American expansionism; another instance of Manifest Destiny – the quasi-religious belief that the United States was destined to span the continent. It was an idea that would, in just a few decades, cause US sights to turn towards Alaska and Greenland.

The 1848 treaty brought peace, but just nine days before it was signed, gold was discovered in California – this soon-to-be American territory. The discovery came on 24 January 1848 when James W Marshall found flakes of gold while supervising construction of a sawmill on the American River in the Sierra Nevada foothills. This began an economic boom that shaped the next decades of American prosperity. For Mexico, the timing couldn’t have been crueller.

According to historian Paul Gillingham, that short period of days between the signing of the treaty and the discovery of gold marks one of the most consequential turning points in North American history: as much as it was actively a disaster for Mexico, it put rocket fuel beneath the rise of the United States.

Speaking on the HistoryExtra podcast, Gillingham situates this moment within a much longer story of imperial inheritance, fragile statehood and American expansionism.

Mexico’s colonial inheritance

When Mexico achieved independence from Spain in 1821, it inherited an enormous territorial claim. On paper, the new republic stretched “from Oregon in this giant triangle, to Texas and then down to Guatemala,” as Gillingham puts it. These borders were the legacy of Spanish colonial administration rather than the result of effective Mexican control.

After 1848, roughly half of this territory passed to the United States, as stated under the Treaty of Guadalupe Hidalgo – which marked the end of the Mexican-American War.

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Almost immediately, the loss became central to Mexican politics. Nationalists claimed to have lost half of their entire national territory.

But Gillingham urges caution. “This isn’t really true,” he explains, because “no one really owned the territory much above southern California. It was the indigenous peoples’.”

Large areas of what would later become the American southwest were sparsely settled by Mexicans and only weakly integrated into the Mexican state. Real power rested with Indigenous nations rather than officials in Mexico City. In practical terms, this wasn’t territory governed by Mexico in a meaningful sense.

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Nevertheless, it was a vast ocean of land that would, within days, change the trajectory of North America.

Manifest Destiny and American expansionism

The Guadalupe Hidalgo treaty was the outcome of the Mexican–American War, a conflict sparked by the annexation of Texas by the USA in 1845. It was driven by a broader US ambition to expand westward, represented in the ideology of Manifest Destiny. By the 1840s, this had become a powerful political idea.

Mexico entered the war divided, riven by frequent changes of government and weak institutions. The United States entered wealthier, more unified and increasingly industrialised. It wielded this power effectively, and the Mexicans were soon on the back foot.

By the time peace negotiations began, US forces had occupied much of northern and central Mexico, including Mexico City itself. Mexico’s bargaining position was effectively gone. As Gillingham puts it, “their national state was destroyed.”

Why, then, did the United States not take more?

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Mexico’s mountainous interior was difficult to occupy and historically favourable to local resistance. “Like anywhere else around the world, people who live in mountains tend to be good at war and don’t like other people treading on their turf,” says Gillingham. Any further American advancement would come under heavy losses and diminishing returns.

“The further-sighted people realised they had got about as much as they would ever get,” Gillingham explains.

So, Mexico signed the peace treaty, says Gillingham, because it lacked the capacity to continue fighting. The United States signed because it had secured its strategic aims without overextending itself.

But even with Manifest Destiny propelling them forward, Gillingham recounts that some on the American side felt conflicted about taking so much Mexican land. Gillingham explains how one American negotiator, Nicholas Trist, was “wracked by guilt at what he was doing” and attempted to derail the agreement so that Mexico might retain more territory. His attempted self-sabotage failed.

California was now American.

The nine days

Nine days before the treaty was signed, gold was discovered near San Francisco. It triggered a boom in capital that reshaped the United States. The gold rush accelerated westward settlement, financed new railroads, underpinned industrial growth and helped propel the US toward its eventual status as a superpower.

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Had California remained Mexican, what might have happened?

Gillingham says, “within two years, the gold production from San Francisco would’ve paid off the entire Mexican national debt.”

Additionally, he explains that “Mexico had all these plans for national banks, development banks, for industrialisation, which never happened because the capital wasn't there.”

He describes a tentative “alternative reality” in which Mexico, having kept hold of its territory, would have unlocked the capital to make those plans a reality.

This is why Gillingham states that the loss to Mexico was more than territorial. “It wasn’t so much what was already there, as what would be there, that Mexico lost,” he says.

“You’ve got a loss of national identity, you’ve got a loss of territory, you’ve got a loss of the future.”

A pivotal nine days separated the discovery of gold from the signing of the treaty, marking the transition from war to a new phase of American expansion and the beginning of extraordinary growth. For Mexico, those same nine days encapsulated the loss of its land, and the loss of a potential future.

Paul Gillingham was speaking to Spencer Mizen on the HistoryExtra podcast. Listen to the full conversation.

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