Mexico City • This spring, hundreds of thousands of visitors to Mexico City’s Museum of Modern Art experienced the ardor of two of Frida Kahlo’s masterpieces in a temporary exhibit: “Diego on My Mind,” depicting Kahlo with Diego Rivera’s portrait emblazoned on her forehead like a third eye, and “Self-Portrait with Monkeys,” showing her surrounded by four monkeys amid dense tropical foliage.

It was a rare chance for Mexicans to see these two celebrated paintings at home. For about two decades, they have largely been either out of sight or on display overseas. Soon after the exhibition closed, the two Fridas were on a plane to Spain — with no sense of when they would return for good.

These paintings, alongside works by Rivera, Rufino Tamayo, José Clemente Orozco and other artists who have helped define Mexico’s modern visual identity, belong to the Gelman Collection, perhaps the greatest private collection of 20th-century Mexican art in the world.

Earlier this year, the Spanish bank Santander announced that it had taken over these paintings’ long-term management and would relocate them to a cultural center in Spain, despite Mexican heritage laws intended to keep works of major cultural importance in the country. The news caused an uproar in Mexico.

The fight about these priceless works — where they should remain and who can have access to them — is really a battle over cultural patrimony and national sovereignty, and it’s one the Mexican government seems to have forfeited. Instead, I, alongside other artists, scholars, lawyers and journalists, have been fighting to keep the collection where it belongs.

I belong to a generation of Mexicans that grew up with Frida Kahlo fatigue, weary of seeing her face plastered everywhere, on keychains, on mugs, on votive candles. She seemed to belong less to us than to the foreigners who loved her more. Then, in 2010, I happened upon a retrospective of her work in Berlin, where I encountered her Gelman paintings for the first time. I had never seen them in person, and they were extraordinary. I understood that Kahlo hadn’t become foreign at all; she’d simply painted herself, and a nation, onto the world stage. As she became universal, Mexicans became universal, as well.

But Kahlo, despite her fame, wasn’t all that prolific: She created only around 200 works, of which roughly half are in Mexico and only a handful are owned by the Mexican state. Recognizing their national importance, a 1984 presidential decree sought to safeguard public access to the few Kahlo works that remained in Mexico by designating them artistic monuments and barring their permanent export. Alongside other protected works, they can leave the country only temporarily.

Nearly 20 of those Kahlos ended up in the Gelman Collection. Having fled from Europe to Mexico amid the upheavals of World War II, Jacques and Natasha Gelman made a fortune producing films during the golden age of Mexican cinema, at the height of Frida and Diego’s fame in the 1930s and 1940s. As the couples became good friends, the Gelmans bought their work, adding it to a growing personal collection that soon spanned hundreds of paintings, drawings and more.

The Gelmans clearly imagined the collection’s future as public. Understanding its national significance, Ms. Gelman stipulated in her will that the collection was to remain intact and accessible to the public in a museum or cultural center in Mexico. When she died, in 1998, the executor of her will, the American curator Robert Littman, released a statement in which he acknowledged his responsibility for ensuring that the collection be “exhibited in its entirety at an institution in Mexico, which has to be determined.”

But the collection, under Mr. Littman’s supervision and assisted by successive Mexican governments, instead embarked on a long sequence of mostly international exhibitions, generating revenue for a foundation established by Mr. Littman. Over the years, Kahlo’s popularity continued to grow, and the collection continued to gain global recognition — and market value. Mr. Littman became embroiled in a series of legal disputes surrounding the collection, amid questions over Ms. Gelman’s mental fitness at the end of her life and whether her final wishes were properly carried out. (Those disputes were ultimately resolved in his favor. Mr. Littman’s lawyer did not respond to requests for comment.)

Then, in its January announcement, Santander revealed that it would be managing part of the collection, including 10 Kahlo paintings, which it said had already been sold to Marcelo Zambrano, a real estate developer from the northern city of Monterrey, in 2023. The whereabouts of the rest of the collection are unknown.

Per an agreement between Santander and the Zambrano family, the works would be used as collateral for a loan worth at least $150 million to Mr. Zambrano and have a “permanent, but dynamic” presence at the Santander Foundation’s new museum in Spain, an ocean away from their original home. (In a statement, the Santander Foundation said that the arrangement was “in full compliance with Mexican law.”) Some of the greatest works of art ever made in Mexico are being held, quite literally, as security by a foreign bank.

Even more scandalous is the fact that the Mexican government, which is responsible for safeguarding such monuments, facilitated the operation. Just one day before the bank approved the loan, the head of Mexico’s National Institute of Fine Arts and Literature signed a renewable five-year agreement with Santander and the Zambranos to oversee the international display of the protected works, in an arrangement a Santander official described as “flexible.”

Although Mexican government and bank officials insist these paintings will return to Mexico in 2028, there is no legal obligation to keep them in the country long term, nothing to prevent authorities from issuing one temporary export permit after another. President Claudia Sheinbaum’s government, which presents itself as the defender of Mexico’s sovereignty, seems perfectly willing to surrender a matter of national concern to foreign interests.

This shouldn’t come as a surprise to observers of Morena, Mexico’s governing party. For much of the 20th century, the state operated as Mexico’s greatest cultural patron — building museums, commissioning art and assembling collections. But Morena has made clear, through routine slashes of the country’s arts and culture budget, that it is not interested in playing that role. The International Committee for Museums and Collections of Modern Art recently expressed “serious doubts” about the Mexican state’s “ability to guarantee the conservation and access to heritage, even if there are laws specially designed to safeguard it.”

If Mexico allows these works to be permanently exiled across the Atlantic, it will be diminished in the eyes of the world. Is our history so easily fragmented and reduced to financial assets? Are we so powerless to protect our own heritage? How can we claim geopolitical strength if we are incapable of keeping our own treasures?

Kahlo’s story doesn’t have to end this way. Mexico’s government might have abdicated its role as a cultural steward, but Mexico’s civil society can and must step up and provide the resources, institutions and imagination necessary to protect works of national importance.

Frida Kahlo took what was most private — her body, her pain, her love, her obsessions — and painted it into works in which millions came to recognize themselves. She belongs to the world, indeed. But she belongs, first and foremost, to Mexico.

Francisco Berzunza is a Mexican historian. This article originally appeared in The New York Times.