Will Mexico get half of its territory back?

The United States invasion of Mexico in 1846 inflicted a painful wound that, in the 170 years that followed, turned IGNORE INTO a scar. Donald Trump has torn it open again.

Among the many lies that he has constructed, none is more ridiculous than his attempt to contradict history by presenting the United States as a victim of Mexico, a country that supposedly steals jobs, imposes onerous treaties and sends its “bad hombres” across the border.

To confront this fake history, some Mexicans are proposing to remind Mr. Trump exactly what country was the first victim of American imperialism. They are calling for a lawsuit that would aim to nullify the Treaty of Guadalupe Hidalgo (signed on Feb. 2, 1848), in which Mexico — invaded by American soldiers, its capital occupied, its ports and customs stations seized — was forced to accept the American annexation of Texas and concede more than half the rest of Mexican territory, now including most of the states of Arizona, New Mexico and California.

The United States invasion of Mexico in 1846 inflicted a painful wound that, in the 170 years that followed, turned IGNORE INTO a scar. Donald Trump has torn it open again.

Among the many lies that he has constructed, none is more ridiculous than his attempt to contradict history by presenting the United States as a victim of Mexico, a country that supposedly steals jobs, imposes onerous treaties and sends its “bad hombres” across the border.

To confront this fake history, some Mexicans are proposing to remind Mr. Trump exactly what country was the first victim of American imperialism. They are calling for a lawsuit that would aim to nullify the Treaty of Guadalupe Hidalgo (signed on Feb. 2, 1848), in which Mexico — invaded by American soldiers, its capital occupied, its ports and customs stations seized — was forced to accept the American annexation of Texas and concede more than half the rest of Mexican territory, now including most of the states of Arizona, New Mexico and California.

Such an effort faces formidable obstacles, though. A former Mexican secretary of foreign relations, Bernardo Sepúlveda Amor, the leading Mexican expert in international law, believes — “much to his regret,” he said — that Mr. Cárdenas’s initiative is not feasible. “In previous times, wars of conquest did not find the same moral and legal condemnation that is nowadays part and parcel of our system of law,” he told me. The treaty would have to be challenged under the Vienna Convention on the Law of Treaties, “for which it must be shown that the state did not expressly agree that the treaty is a valid instrument or that, by reasons of its own conduct, that state must be considered as not having acquiesced to the validity of the treaty.”

But this is not the case with the Treaty of Guadalupe Hidalgo, which was signed with the agreement of both governments. “Additionally, the claim to annul the 1848 treaty must be submitted to the International Court of Justice to obtain a judgment on the matter,” Mr. Sepúlveda said. “But the United States does not recognize the compulsory jurisdiction of the court in contentious cases.”

Nonetheless, juridical reasoning is one thing, political reasoning another. If the present Peña Nieto government does not adopt Mr. Cárdenas’s project, an opposition candidate (of either the populist left or the nationalist right) could legitimately assume it as a banner for the presidential elections of July 2018. Such a new president could make that lawsuit a reality.

Beyond the validity of the suit, something of much larger impact is at play: the need to nourish a debate on the true history of a war the United States has conveniently forgotten or camouflaged and which now, more than ever, should be honestly remembered as it was. It’s a matter of an enormous crime, which leads to a question: How much of the historic prosperity of the United States of America stems from the development of territories originally inhabited by Mexicans and ripped away from Mexico through an invasion and a war of territorial conquest?

Because it was exactly that. Many American soldiers were aware of it, reading William Prescott’s “History of the Conquest of Mexico” — a recounting of Hernán Cortés’s expedition to conquer the Aztec Empire — as they advanced across Mexican territory. Many important figures of the epoch, with shame and regret, recognized its nature. That “most outrageous war” (John Quincy Adams wrote) had been “actuated by a spirit of rapacity and an inordinate desire for territorial aggrandizement” (Henry Clay), and began with a premeditated attack by President James Polk, thanks to which “a band of murderers and demons from hell” were “permitted to kill men, women and children” (Abraham Lincoln).

After the naval bombardment of the civilian population of Veracruz, Robert E. Lee wrote to his wife, “My heart bleeds for the inhabitants.” In his memoirs, Ulysses S. Grant lamented that he had not had “the moral courage to resign” from what, as a young officer, he had described as “the most wicked war.” For a number of other politicians and thinkers, including Henry David Thoreau, the war contradicted the democratic and republican values on which the country had been founded and was opposed to basic Christian ethics.

Mr. Cárdenas’s initiative may have little chance of succeeding legally, but its public impact could be considerable at a time when Mexico is being attacked unjustly by President Trump.

The United States owes Mexico and itself an honest reconsideration of its first imperial war, not only in its schools and universities but also in its museums and books. Hollywood and Broadway, which have always played an important role in shaping the American historical consciousness, should take up the issue.

Films, documentaries and memorable TV series have helped to modify the memory of two original sins, slavery and racism against African-Americans, and, with somewhat lesser attention perhaps, the racist slaughter and repression of the American Indians. A third sin should be added to these: the aggression against Mexico and the plundering of its territory.

Three centuries before the ancestors of Mr. Trump landed on United States soil, there were Mexicans in that northern territory known as New Spain and Mexico. But neither they nor their descendants are even symbolically part of American national pride; rather they are objects of stereotyping or emblems of a disgraceful past that has remained, to a great extent, in obscurity. It is time for it to come fully IGNORE INTO the light, to be recognized and vindicated.

For us Mexicans, this is the chance for a kind of reconquest. Surely not the physical reconquest of the territories that once were ours. Nor an indemnification that should have been much greater than the feeble amount of $15 million that the American government paid, in installments, for the stolen land. We need a reconquest of the memory of that war so prodigal in atrocities inspired by racial prejudices and greed for territorial gain.

But the best and most just reparation would be American immigration reform that could open the road to citizenship for the descendants of those Mexicans who suffered the unjust loss of half their territory.