Setting up effective, local security forces

[Dayuhan]

The Filipinos come more close, but at least the WW2-period Filipino troops were motivated by a promise of independence and thus again fighting for their people, not really for the Americans AFAIK.

True of the WW2 period, but well before independence was promised the US was training Filipino units and deploying them for internal security functions, just as most imperial powers did.

What's remarkable in the case of U.S. troops is that they don't form U.S.Army units with 80-90% foreigners from the region. It's really not that hard, as evidenced by the ease of how European powers did this during Imperialism times. See the German Askaris; German officers surely had no experience in creating such a force, yet built a formidable one in East Africa with IIRC initially Sudanese warriors.

Just imagine; rotation would be limited to about 20% of the total force, deployed U.S. personnel could be cut by two thirds and the actual force available in-theatre would still be larger and have enough boots on the ground to dominate most of the places that are now effectively without Western control.

What you're missing here is that the whole idea of recruiting locals directly into the armed forces of a foreign power is only possible is the foreign power rules the area. You can't do it if there's an even nominally sovereign local government in the picture. The US could and did form such units in its colony in the Philippines. It couldn't and didn't and hasn't in Vietnam, Iraq, or Afghanistan because these are not direct imperial ventures where the US is setting up to rule, they are nominally sovereign states with their own governments and armed forces.

Either you're a colonial power, in which case you can and will take direct control of indigenous armed forces, or you're not, in which case you can't and won't. Can't have it both ways.