Answer: Tangier, Morocco 

The United States Federal Government maintains a register of districts, sites, buildings, structures, and objects deemed worthy of preservation. The register includes everything from theaters to parks to statues, and even burial plots. The one thing that ties all the entries in the register together is that they are all located within the borders of the United States.

All of them, that is, save for one. The Tangier American Legation, a building in Tangier, Morocco, is the first piece of public property owned and maintained by the United States Government after the U.S. became a sovereign nation. Built in 1821, it housed the United States Legation and Consulate for 140 years (making it the longest occupied diplomatic post in U.S. history) and was later turned into a base of operations for World War II-era intelligence agents.

By the 1970s, the building had fallen into heavy disrepair. A group of American citizens funded a restoration effort, turned it into a museum commemorating the long history of diplomatic relations and friendship between the United States and Morocco, and it was placed on the National Register of Historic Places during the early 1980s.