Trump wants to send troops into Mexico. The land grab of the Mexican-American War makes this ‘politically untenable’

After the United States government celebrated the capture of Nicolás Maduro in Venezuela, President Donald Trump issued warnings about what might come next. “Something will have to be done about Mexico,” he said. “American dominance in the Western hemisphere will never be questioned again.”

The US government has subsequently pushed for US forces to accompany their Mexican counterparts in operations targeting fentanyl production. But the history of US military intervention in Mexico makes the presence of foreign troops, under any pretext, politically untenable.

The arrival of US troops in the past has signalled political instability, suffering for ordinary people, and in the most extreme case, massive loss of territory. Before the one-sided Mexican-American war of 1846–48, most of what is today the states of California, Nevada, Utah, Arizona and New Mexico had been part of the newly independent Mexican republic.

The second Trump government’s realignment of security policies includes an aggressive reboot of the 19th-century “Monroe Doctrine”, which projected influence across the western hemisphere. Trump likes to call the updated version the “Donroe Doctrine”, and it dispenses with any pretense of hemispheric solidarity.

The embrace of this doctrine has people and governments throughout the western hemisphere on edge. Even during so-called isolationist periods in US history, parts of Latin America have faced frequent US military interventions.

Gunboat isolationism

The “Monroe Doctrine” started not with a bold declaration to the world, but with a routine address by President James Monroe to the US Congress in 1823. Throughout the Americas, newly independent states were emerging from European rule. The doctrine stated that any attempt to oppress or otherwise control newly independent states would be viewed as “the manifestation of an unfriendly disposition toward the United States”.

This doctrine is often seen as an expression of US isolationism. Suspicious of the European powers, the US sought autonomy to develop on its own. It would keep out of European affairs (which it was already largely doing), but wanted the same from European imperial powers – some of which were already in decline.

The Monroe Doctrine was aspirational at best. The US lacked the political appetite or military capacity to actively protect its regional neighbours. Two years after Monroe initiated the policy, French warships returned to the capital of independent Haiti, coercing the government into agreeing to pay massive reparations to the former colonial power. The US did not intervene.

When French warships blockaded and then raided the Mexican port of Veracruz in 1838 – seeking exorbitant reparations for losses incurred by French citizens – the US did not intervene. The US did, however, send a ship to police the gulf waters and prevent smugglers from circumventing the French blockade.

Despite the “unfriendly disposition” shown by European gunboat diplomacy, the Monroe Doctrine showed little indication of delivering on its stated hemispheric commitments.

As its territorial ambition and military capacity grew, the US assumed a more active role within the hemisphere. This regional intervention looked a lot like the gunboat diplomacy practised by former colonial powers.

The one-sided Mexican–American War

When Monroe announced his doctrine, the US was made up of just 24 states, plus large expanses of unincorporated territory.

By the 1840s, the US was eager to push westward and acquire new territories. James Polk won the presidency in 1844. The following year, he made good on his pledge to annex Texas, which already claimed independence from Mexico.

He also made diplomatic moves to acquire territories further west by negotiation. He offered US$5 million for New Mexico (which included what today is Utah, Nevada and parts of other states) and $25 million for California. He also prepared land and naval forces, in case he needed to take the territories by force.

The young Mexican state, first established in 1821, was in the midst of political tumult. The government at any particular time faced weak domestic authority, but clear nationalistic demands to preserve the territory and honour of the young country.

When Mexico would not negotiate on Polk’s terms, US troops moved into the coveted territories of New Mexico and then California. Mexican state presence in these areas was weak at best; there was little sustained opposition to the US occupation.

The question of how to formalise these conquests brought the war into the very heart of Mexico. US forces landed at the port of Veracruz (as French forces had done ten years earlier). From there, the invading force fought a series of battles, eventually occupying Mexico City, the political and symbolic centre of the Mexico, in 1847.

The occupation of Mexico brought further negotiations on very different terms. The US acquired the entire claimed territory. It paid US$15 million in compensation (equivalent to almost US$500 million today) – roughly half the amount offered before the war.

Mexico lost 55% of its total territory.

A new threat of US military intervention

While Venezuela has long been one of the main US antagonists in the region, Mexico and the US enjoy a much closer and more productive relationship. The US and Mexican economies are tightly integrated.

In 2025, Mexico became the largest export market for the US. The two governments cooperate across a wide range of issues and later this year, Mexico and the US, along with Canada, will co-host the FIFA World Cup. In the nativist world view of Trump and his close advisors, however, Venezuela and Mexico are seen as two sources of the same core issues: migration and illicit drugs.

President of Mexico Claudia Sheinbaum has so far dealt with pressure from the Trump government by demonstrating that Mexico can handle illicit drug trafficking on its own. Her government has ramped up policing and intelligence operations, and touts its successes – in terms of decreasing homicides, and increasing arrests and drug seizures.

Trump and his advisors, however, are committed to military solutions – from the raid in Venezuela to sharing videos of illegal strikes on civilian boats – in part as a way of projecting dominance in the region.

This is entirely the wrong approach. Successive Mexican governments used the military to target illicit drug trafficking – backed by US funding, US military training and US military hardware – without any real success. Violence worsened, but illicit drug trafficking continued.

The US government is, for now, committed to its “Donroe Doctrine”. The violence and militarism of this approach might seem new, but it is little more than the Monroe Doctrine stripped of the veneer of isolationism.

The original doctrine only ever spelled isolation from Europe. For the people of Mexico and much of the Americas, it was always a doctrine of US self-interest and military intervention.

Philip Johnson does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.