Lonnie G. Bunch III will step down from his position as head of the sprawling Smithsonian Institution by the end of the year, he said Tuesday. He will depart at a vulnerable time for the institution, which includes 21 museums, 20 libraries and the National Zoo. While it has traditionally operated independently of the party in power, the Trump administration has repeatedly criticized the Smithsonian’s presentation of America’s past. Last week, the Interior Department threatened to cut off support, alleging that Smithsonian facilities present a “radical ideology” that weakens “shared American identity.” Bunch, who has worked at the Smithsonian for 38 years, said in a statement that “It is with very mixed emotions and heartfelt gratitude that I announce my retirement.” “I have spent nearly half of my life working at and being part of the Smithsonian family, and what a privilege it has been. There is no place like the Smithsonian — it has been one of the great loves of my life, and I am...