But, to be fair, we didn't need to spend most of our military time chasing redskins around the plains....
Gen Custer at Little Big Horn: "Hmm, bugger. There are rather a lot of them, aren't there chaps..."
We needed a navy to keep out various invaders rather more than we did an army to subjugate our natives - so our navy was the senior service. Whereas the US was once actually quite keen on letting people in by sea, but needed an army first and foremost to fight its way west to the Pacific.