Very interesting question(s)!

To begin with your specific one about the use of “Indian Country” in U.S. military manuals: I am not aware of any specific historical studies showing that to be the case. The point of reference is usually the same story about Richard Neal as described by Dunbar-Ortiz. Unfortunately, the book leaves out the important part of the story involving military manuals. Richard Neal’s use of the term “Indian country” in relation to a military operation in Kuwait sparked the response of Native American Veterans condemning it. Pentagon spokeswoman Michelle Rabayda then responded to the condemnation by saying that “Indian country” had no official definition in military manuals. This story dates from 1991 and the Associated Press was one of the news outlets covering it (link in the notes below). In relation to military manuals, the same event is referred to by Stephen W. Silliman (2008) and Al Carroll (2008). If “no official definition” was the actual phrasing used, then that’s very different from a term not being included in military manuals. Therefore, Dunbar-Ortiz probably interpreted the wording “no official definition” along the lines of “Indian Country” actually being used in military manuals, but without a specified definition attached to it. By contrast, Silliman offers a completely different interpretation of it by saying that the term ‘was not part of any official manual or training’. Silliman based this on a more obscure newspaper article that I have no access to, but luckily Silliman’s article is open access, so you can have a try at finding the article he cites. So in terms of your specific question about military manuals, I am not aware of any research on that specific topic, and the statement by Dunbar-Ortez is probably based on the Neal incident.

When historians don’t use footnotes for their claims, it is often due to the statement or the message of the statement being considered general knowledge. My impression is that the decision not to expand on the history and the widespread usage of the term was made on that basis. (I am happy to have further discussions about what counts as “general knowledge” and the misuses of (lack of) notes/references, but I don’t want to sidetrack from the question that you asked.) There is a comprehensive and reliable open-access piece on the recent history and usage of the term “Indian Country” in the military can be found in Stephen W. Silliman’s article titled: The “Old West” in the Middle East: U.S. Military Metaphors in Real and Imagined Indian Country. I mention this because in the last paragraph, you write ‘So I'm left seeing that clearly the phrase is at least likely used unofficially’, from which I infer that you are interested in knowing just how widespread the use of that term is.

Silliman’s article notes that there hasn’t been a systematic history of how this specific usage of the term emerged within the military. (Though this was published in 2008, so someone might have done so since.) The article begins by discussing the history of the term since the Vietnam War. He points to the works of four other historians who noted the use of the term in war related newspaper coverage, popular books and films in the 1970s. In the notes at the end of my post, I added references to them, with the exact page numbers that Silliman also notes. Silliman also points to the work of historian David Stannard, who argued that the ‘official government language’ used the same term. Silliman himself quotes an exchange between a congressman (John Seiberling) and Captain Robert B. Johnson from the transcripts of the war crime hearings following the 1971 Mỹ Lai Massacre:

“Johnson: Where I was operating I didn’t hear anyone personally use that term “turkey shoots”. We used the term “Indian Country”.

Seiberling: What did “Indian Country” refer to?

Johnson: I guess it means different things to different people. It is like there are savages out there, there are gooks out there. In the same way we slaughtered the Indian’s buffalo, we would slaughter the water buffalo in Vietnam.”

Although Silliman cites a no-longer-accessible website as a reference to this, you will find the transcript in the Dellums Committee Hearings on War Crimes in Vietnam, which is accessible via Archive.org (I added a reference to it below). Silliman then goes on to use examples for the usage of the term dating from the Gulf War (the same story about Richard Neal), Iraq and Afghanistan. The article also includes a table of quotes showing 13 instances of newspapers, media outlets, and comments using the term in relation to military operations. The list included outlets such as Wall Street Journal, The New York Times, The Los Angeles Times, and Fox News. I think the widespread use of the term is important to mention, as coupled with the interpretation of the Pentagon response (“no official definition”) in the Neal case, it can be used as further contextual evidence for the presence of the term in military manuals.

What I personally think would be more fascinating to look at is why the same story and statement appears twice in the book. First, on page 57 and then on page 193. In the first instance, Dunbar-Ortiz refers only to “Indian Country” appearing in “military training manuals”. In the second instance, she refers to both “Indian Country” and “In Country” appearing in military training manuals. She further mentions that the “In Country” was derived as a shorthand from “Indian Country” during the time of the Vietnam War. This difference is interesting, as the first one appears in her article written in 2004 (link below), while the second one seems to indicate further research into the use of “In Country”. So this raises a third possible answer to your question, which is that while “Indian Country” is not defined (or used) in military training manuals, its derivative “In Country” is being used so. (Remember here that the Pentagon’s response only mentioned “Indian Country” and not “In Country”, and specifics matter when it comes to official communications).

What I have written is just a very brief snippet of Silliman’s article. It is free to read, so I’m sure your curiosity and eagerness to check notes/references will lead you to read the relatively short article. I also hope my answer explains on what grounds I think that Dunbar-Ortez (and/or the editors) considered not including further examples/references to the usage of the term. You are certainly starting to think like a historian, which is always great to see! Following references and footnotes is where the extent of historical research often gets revealed. So it is always worth reading the notes/references as you go along regardless of the author.

As for your final question: “how do I go about not even necessarily checking the claims made in this book, but simply finding more reading on them?” - It depends on how much you want to engage with a text, and I don’t think that question should be conflated with what is bad or good history. There is definitely an interesting question here, which is about why you should trust historical research or how reliable historical research is generated, but that’s beyond the scope of my post.

References and Notes:

Robert Imrie, ‘Tribes Angered By General’s Reference to Enemy Land as ‘Indian Country’’ - 21 February 1991, Associated Press. Accessible via this link: https://apnews.com/article/ce150feb55e4a9058c307295efc07f4a

Stephen W. Silliman, ‘The “Old West” in the Middle East: U.S. Military Metaphors in Real and Imagined Indian Country’, - American Anthropology, vol. 110, issue 2, 2008, pp. 237-247. Accessible for free via this link: http://www.faculty.umb.edu/stephen_silliman/Articles/oldwestinmiddleeast.pdf

Books mentioning the usage of the term in Vietnam War related materials:

Carol Burke, Camp All-American, Hanoi Jane, and the High-and-Tight: Gender, Folklore, and Changing Military Culture. (Boston: Beacon, 2004). - page 109

Richard Drinnon, Facing West: The Metaphysics of Indian-Hating and Empire-Building. (New York: Schocken, 1990 [1980]). - page 368

Tom Engelhardt, The End of Victory Culture: Cold War America and the Disillusioning of a Generation. (Amherst: University of Massachusetts Press, 2007, revised edition) - pages 175-259

David Espey, America and Vietnam: The Indian Subtext. Theme issue, “Uprising: The Protests and the Arts,” David Landrey and Bilge Mutluay, eds. Journal of American Culture and Literature, 1994: 128– 136.

David Stannard, American Holocaust (Oxford: Oxford University Press, 1992) - page 251.

Roxanne Dunbar-Ortiz, ‘Indian Country’, Accessible via wayback machine: https://web.archive.org/web/20080218180259/https://www.counterpunch.org/ortiz10122004.html

Citizens Commission of Inquiry (ed.), The Dellums Committee Hearings on War Crimes in Vietnam (New York: Vintage Books, 1972) - see pages 52-53. Accessible via this link: https://archive.org/details/dellumscommittee0000unse