Curmudgeon, you're correct when it comes to the size of the Army prior to World War I. State Volunteer units were involved in the early stages of the Philippines, but most of them were gone by about 1902.
The Army never really "grew" to deal with the Indian Wars. It expanded slightly after the Civil War to deal with the greater expanse of territory it needed to cover, but remained at more or less the same strength from 1866 until 1898. In fact, it shrank between 1866 and 1870 (mainly in terms of infantry regiments - cavalry remained constant at 10 regiments). It also never really took in Volunteer units...in fact they were normally resisted as being more trouble (and expense) than they were worth. There are exceptions, but they were not the norm by any means.
It has been argued that Frontier duty was really more like constabulary duty, which might explain why some Army officers performed well in the Philippines. Linn's work has shown that this wasn't always the case, and it certainly doesn't explain the Marine Corps' record in this area. Bickel's "Mars Learning" is really worth reading when it comes to studying how "lessons learned" may or may not have influenced doctrine in this area.
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