a lot of them are joining the military because it's the only viable option for a decent future; ESPECIALLY if live on a bad rez.
the tohono o'odham have had some luck when the state (AZ) did some capital investments in the form of business loans that created jobs (casinos); i worked with someone who benefited from it and she told me all about it back in the late 1990's. (i bet the state has changed its tune since it voted for trump the last 3 times and, last i heard, all of the rez's in the state are struggling rn).
so maybe a message of jobs or other opportunities besides the military would be helpful?