In a federal lawsuit filed on Friday, collector Ronald S. Lauder revealed himself as the owner of a $25 million collection of World War II memorabilia that includes around 3,500 books, 500,000 photographs, 100 uniforms, 1,650 posters, and a Russian anti-tank cannon, among other objects.

The suit, filed against the International Museum of World War II in Natick, Massachusetts, claims that the institution has refused to ship to him in a timely fashion the collection, which he acquired from the museum’s founders. The suit, which was filed in the Southern District of New York by Ellenville, LLC, a company owned by Lauder, claims that the museum is “stonewalling” the billionaire collector.

According to the suit, Lauder had an agreement with the Museum of World War II for it to show the collection of military-related books, photographs, and other memorabilia and artifacts through the end of the year. However, the suit alleges, Lauder has asked to take the work off view early and the museum has attempted to keep him from doing so. The suit claims that Lauder has the right to remove his holdings from the museum at any time.

Judd Grossman, a lawyer representing the museum, declined to comment, and referred ARTnews to a letter sent by his firm to Lauder’s legal team. In that letter, Grossman calls Lauder’s demands “arbitrary” and “unreasonable.”

A representative for Lauder declined to comment.

A contract included in the filing says that Lauder made the purchase in 2018 from museum founders Kenneth W. Rendell and Shirley McNerney Rendell. The website for the museum, which was founded in 1999, bills those holdings as the “most comprehensive collection” of World War II objects of any museum in the world.

Lauder, who founded the Neue Galerie in New York in 2001, is a prodigious collector of German Expressionism, antiques, postwar German and Italian art, and contemporary art. Along with his wife, Jo Carole Lauder, he has appeared on the ARTnews Top 200 Collectors list each year since 1998, and he in 2006, bought Gustav Klimt’s Portrait of Adele Bloch-Bauer I (1907) for $135 million—a sum that was, at the time, the highest price ever paid for an artwork at auction.

Documents accompanying the suit claim that the collection Lauder purchased was to stay on view at the museum through December 31 of this year. According to the suit filed on Friday, Ellenville terminated the loan in mid-April, but was informed by a lawyer for the museum that the holdings could not be retrieved until December 2.

“Ellenville is prevented from ascertaining the integrity and completeness of its collection, and displaying the artifacts it owns publicly in locations of its choice,” the suit reads. Lauder is seeking the return of his collection, and is not asking for monetary damages.