Hundreds of thousands of visitors to Mexico City’s Museum of Modern Art recently saw two Frida Kahlo masterpieces in a temporary exhibit: “Diego on My Mind” and “Self-Portrait With Monkeys.” It was a rare chance for Mexicans to see these celebrated paintings at home. For about two decades, they have largely been out of sight or on display overseas. Soon after the exhibition closed, the two Fridas were on a plane to Spain.

These paintings, alongside works by Diego Rivera, Rufino Tamayo, and José Clemente Orozco, belong to the Gelman Collection, perhaps the greatest private collection of 20th-century Mexican art. Early this year, Spanish bank Santander announced it had taken over the collection’s long-term management and would relocate it to a cultural center in Spain, despite Mexican heritage laws. The news caused an uproar in Mexico.

The fight over these priceless works is really a battle over cultural patrimony and national sovereignty, one the Mexican government seems to have forfeited. The author, an artist and scholar, has been fighting to keep the collection in Mexico. They grew up with Kahlo fatigue, seeing her face on keychains and mugs, but a 2010 retrospective in Berlin revealed the paintings’ extraordinary power.

Kahlo was not prolific, creating only around 200 works, with roughly half in Mexico and only a handful owned by the state. A 1984 presidential decree designated them artistic monuments, barring permanent export. Nearly 20 of those Kahlos ended up in the Gelman Collection. Jacques and Natasha Gelman, film producers in Mexico’s golden age, befriended Kahlo and Rivera and amassed the collection. Natasha’s will stipulated it remain intact and accessible in a Mexican institution, but executor Robert Littman instead sent it on international tours, generating revenue for his foundation. The collection’s fate remains disputed.