The movie described in the trailer and a book (The Monuments Men: Allied Heroes, Nazi Thieves and the Greatest Treasure Hunt in History by Robert M. Edsel) describe the efforts made by an American led Allies team to protect art treasures during World War II. The men involve risked their lives (and two died in the effort) to keep world heritage safe during combat. The effort continued to recovering the artistic treasures that had been appropriated by the Axis forces as spoils of war, or that had been confiscated as what they considered "degenerate art".
In a radical departure from the practice of other victors in other wars, the western Allies after World War II chose to return the works of art that they had managed to rescue to their countries of origin.
Of course, after the War the western Allies led in the creation of UNESCO, which had among its early functions helping the museum community in Europe to rebuild and reorganize. UNESCO quickly established international conventions to protect cultural heritage threatened by war, to prevent the international trafficking in stolen cultural heritage, and to protect World Heritage.