How to Hide an Empire: a short history of the greater United States. It includes an excellent discussion of how technological advancements in the 20th century (

How to Hide an Empire: a short history of the greater United States. It includes an excellent discussion of how technological advancements in the 20th century (such as synthetic rubber, airplanes, and radio) changed the physical structure of empires, with the result of the US empire existing primarily in bases scattered around the world rather than the large swaths of conquered territories that had traditionally characterized empires.