A stunning book: erudite yet accessible. Immerwahr's central move is to change the perspective(s) through which we consider US imperialism.
Specifically, he argues that overseas territories and land masses matter. In the early 19th century, American westward expansion (and the displacement of Native Americans) emerged from a booming white settler population's need for more land. Once all that land was settled, policymakers started grabbing and annexing overseas territories, much in the manner of British and French empires — e.g. seizing the Spanish-controlled Philippines at the end of the Spanish-American War (to the great dismay of Filipinos, who thought the US was going to liberate them). But after World War II, Immerwahr shows, when the US had the possibility to annex and keep even more territory than ever, they didn't. Instead, the US gave up territories and instead swapped out an empire of territories for an empire of bases, specks, leased land, and other small places on the map whose legal status is ambivalent and where the US Constitution does not equally apply.
The empire of bases and specks — the "pointillist empire" — works because the US hasn't needed the large footprint and strategic resources of a "traditional," territorial empire. World War II supercharged American industry—the peacetime application of wartime technologies like radio, plastics, medicines, etc., allowed Americans to have a hand in global affairs without the upkeep.
Immerwahr's account suggests a certain unaniminity of interests, like there was broad political consensus that the US should trade in the lumbering apparatus of "old" empire for the pointillism of globalization. But was there backlash? What is the intellectual or cultural strain that helps explain today's delusional talk of annexation? Immerwahr shows us why the US doesn't annex territory today; not only are such land grabs out of the pale in our liberal international order, but they are also unneeded when the US has so many other avenues to secure its interests.