The introduction talks about how we "mainlanders" talk about "Pearl Harbor" on "December 7, 1941," despite the fact that the Japanese attacked the much larger US territory of the Philippines at the same time... where it was already December 8.
The book is full of fascinating stories that I was only dimly aware of. Immerwahr shows how things look different from the provinces, and how the federal government has been able to take advantage of the murky legal status to act like other imperial powers (which is to say beastly).
After the Second World War, the US was in a position to greatly expand its empire but instead started divesting itself of many existing territories. Immerwahr argues that empires in the traditional sense are unnecessary in the modern world, where you don't need to control territory to secure trade routes or fight wars. He doesn't note it, but the other imperial powers also gave up their colonies after the war; see Postwar.
The only thing preventing me from granting How to Hide an Empire five stars is that the final chapters try a bit too hard to tie all aspects of recent history to our hidden empire. I felt like I had to do more work on my own to see the key points.