Among the most valuable art treasures once held at Friedenstein Castle, a vast baroque palace in eastern Germany, was a series of five oil sketches depicting saints by Peter Paul Rubens that disappeared at the end of World War II.

The castle had long been the home of the dukes of Saxe-Coburg and Gotha, a duchy that lost its powers, titles and much of its land after the 1918 revolution in Germany. The palace complex then became a public museum, filled with the art once owned by the ducal family and operated by an independent foundation.

But representatives of the ducal family returned to the castle in 1945 as American and Soviet forces closed in on the city of Gotha.

According to experts, truckloads of the most valuable art were removed by the family in the last weeks of the war, including three of the five Rubens oil sketches, which were later sold on the market.

One of those sketches, which depicts “St. Gregory of Nazianzus,” is now being returned to the castle and the foundation that operates it by a museum in Buffalo that bought it from a New York gallery in 1952. The Buffalo AKG Art Museum, a successor to the city’s Albright Art Gallery, did not know the work had been misappropriated and illicitly sold, and will receive compensation for the work, under a negotiated agreement, according to a statement from the parties involved in the settlement.

“I am delighted,” said Tobias Pfeifer-Helke, the director of the foundation that oversees Friedenstein Castle’s museums. “Our goal is to restore the historic integrity of the collection — especially its core works, these five Rubens sketches which belong together as a series.”

The oil sketch shows St. Gregory in bishop’s robes standing on a cloud while striking the Devil in the face with his crook. It is one of 22 surviving sketches that Rubens painted on wood in 1620 and 1621 in preparation for the ceiling paintings of the Jesuit church in Antwerp — at that time, the biggest commission he had ever received. The church became a draw for 17th-century tourists because of Rubens’s famous ceiling. But in 1718, it was hit by a lightning bolt and burned down in a matter of hours.

The preparatory sketches are all the more important in the light of that church’s destruction. Unlike the ceiling paintings, which were produced by Rubens’s workshop, the sketches are proven works of the master, Pfeifer-Helke said. “The contract between Rubens and the Jesuits from 1620 is still in existence and it stipulates that Rubens had to paint the sketches himself,” he said.

The Buffalo museum had thought to sell its Rubens sketch via Christie’s in 2021 when provenance researchers at the auction house noted the work’s troubled history.

Cathleen Chaffee, the chief curator of the Buffalo museum, said that the sketch did not fit the institution’s core mission, which is to display contemporary and modern art. The Rubens sketch, she said, “hadn’t been on view for a long time and we didn’t foresee a context in which it would be shown.”

Dirk Boll, who is deputy chairman, 20th and 21st century art, at Christie’s in London, said, “We were able to persuade the U.S. museum not to offer the work at auction, but to have a private sales agreement to repatriate the work to Gotha.”

Under the agreement, reached after more than two years of negotiations, the museum will receive “a low seven-digit figure,” donated primarily by the Ernst von Siemens Art Foundation, which frequently acquires art for German museums.

The parties said the payment is well below the painting’s market value and that any legal claim to the work by the Friedenstein foundation would have faced a number of hurdles.

“Of course it’s clear to us that the American museum bought the work in good faith,” Pfeifer-Helke said. He added, though, that the current holders of such works have “a moral responsibility” to take into account “the historic circumstances of the loss.”

The other two sketches taken by the family — “The Prophet Elijah on the Golden Chariot” and “St. Augustine” — were also sold, and the foundation is attempting to arrange their return, at least on loan. Today, in the Dutch gallery of the palace museum, black-and-white photographs take the places of the three missing paintings. Now, one of these, at least, can be replaced with the original.

The remaining two Rubens sketches at the castle, which depict the theologians St. Athanasius and St. Basil, were seized by Soviet troops who occupied it in 1945. But they were returned in 1958, when in a gesture of friendship to Communist East Germany, 300 train carriages from Moscow and St. Petersburg brought back about 1.5 million objects looted by Soviet troops.

“Friedenstein Castle’s collections suffered more than most German cultural institutions from embezzlement, war losses, and removals to the Soviet Union,” said Martin Hoernes, the general secretary of the Ernst von Siemens Art Foundation. “The recovery of the Rubens sketch will surely pave the way for further returns in which, depending on the circumstances of the loss, the amount paid is not a reflection of the market value but a fair settlement.”

The five oil sketches at Friedenstein Castle had been part of the collection since at least the beginning of the 19th century; one had been there more than three centuries.

The “St. Augustine” sketch is now held by the E. G. Bührle Collection Foundation in Switzerland. The Friedenstein foundation is hoping to secure the long-term loan of the sketch, which was purchased by the collector Emil Georg Bührle in Zurich in 1953, Pfeifer-Helke said. The sketch is currently on loan to the Kunsthaus Zürich.

The fifth sketch, depicting the prophet Elijah, was bought by the Atlanta collector Curtis O. Baer, and is known to have on been loan to the National Gallery of Art in Washington in 1997, Pfeifer-Helke said. But he said it is not clear where it is today.

Though Gotha was initially captured by U.S. forces, under the occupation agreements signed by the victorious powers, the city was later transferred to the Soviet Union’s control. According to the account of a local official after the war, some of the later removals of objects by the ducal family occurred after the American takeover when the last Duchess of Saxe-Coburg and Gotha was able to persuade the Americans to let her transport some of the palace’s treasures west to keep them safe from the Russians.

In a catalog essay written for the museum at the castle by Mirko Krüger, he reports that in 2021 he put some questions to a descendant of the ducal dynasty about the objects sold after the war by the family.

The descendant reacted with “a big shrug,” Krüger wrote, explaining that it had all happened long before he was born and that the family did not keep an archive that explained the events in any detail.

“He was, however, aware,” Krüger wrote, “that the revenue from the sale of artworks was intended to keep the Coburg administration afloat in the postwar era.”