Richard Haass’s book The Picky Eagle: How Democracy and Xenophobia Limited U.S. Territorial Expansion is an excellent book which explains why the United States abandoned territorial expansion and instead built the modern international system which outlawed territorial acquisition.
Spoiler: The U.S. gave up on expansion because it concluded that its neighbors weren’t worth the trouble of annexing and incorporating into the United States. Puerto Rico still isn’t a state.
Invade invite with nothing to show for it.
Yeah, but now we’re so diverse, it doesnt really matter. We get a territorial trade off w more space to roam.
This wasn’t always the case.
The book is about the story of how we avoided becoming diverse by avoiding becoming an empire in the 19th century. This began to change though after the annexation of Hawaii and the Spanish-American War.