In 1890, what constitutes the American Army is very clear. And the US Cavalry is part of the US Army. It was the main military force on the Great Plains and in the West.
No, it was not "mostly Apaches". The US Cavalry fought the Apaches.
And no, it was not the British colonists who were expelled 100 years earlier.
And the Iroquis Nation was indeed 5 tribes. But this was not them.
The US Military, and therefore, the US Cavalry was a branch of the volunteer military service. In that respect it was just like the US Navy.
I have no doubt it doesn't interest you. Mainly because it shows you have been wrong about so many things in this thread.