Basically Comanche and the Lakota and a few other groups had a rather unique approach to warfare and how to treat prisoners of war, compared to a lot of contemporary peoples.
Namely that crap you see post apocalyptic movies would be seen as "generous and compassionate treatment". Like performing C sections on women and then dragging the babies through cactus patches and slow cooking toddlers over open fires while their mothers are forced to watch..ETC ETC. Some of the accounts from survivors and later verified are...
interesting.
I think someone brought up their propensity to crucify kids to wheels and to leave them along major trails.
They fought war, much more like the Mongols did. Their view of what counted for acceptable targets was comparable and given the size of some of their territories and the location, it's easy to see why they were one or two bad decisions away from becoming major world powers.
If they still kept that practice up, into the 20th century or if say the "Comanche Empire" gets integrated into the US but in a way that much of its cultural norms and infrastructure is kept intact...the history century of history would be a lot different.
Or hell it might be entirely alien...If the US had to invest in wars with an organized, more industrialized Kingdom of the Plains..shit might be very different. Innovations in warfare and logistics might come a few decades early by necessity.