One fault is the tendency to attempt to build "mini-me" versions of ones self.
I mean seriously, based on what analysis does Afghanistan need a large conventional army? Has not the traditional and current threat "frustrated" both the Soviet and US versions of the same? This is a country that cries out for a capability much more like the US National Guard; centrally funded and organized, but recruited, trained and employed locally under the control of commanders who answer to their governor until such time that they are federalized. Such a force can also do law enforcement duties, so reduces the need for a large national police force as well.
I get it that Karzai has no trust, and wants total control over security forces to reduce the likelihood of them turning against him; and that we bought into that thinking under the guise of "warlords are bad, so no militias because militias = warlords."
Meanwhile, even the leadership of a great general like LTG Caldwell and the huge push of resources and emphasis on turning s*** into shinola with the ANSF is not working (though with some notable bright spots, such as the Commandos and SF being developed separately by USSF that operate with great effectiveness and are becoming more and more independent in planning and leading their own operations; and some forces developed by British SOF, but those are more seen as local augments to British units, following more of a colonial model).
One also avoids the many problems associated with large standing armies in peace (how to pay, effect on neighbors, vast mobs of unemployed soldiers all at once upon de-mob, etc). The National Guard/militia model avoids this, as the men never really left their villages or jobs and stay on the books at reduced cost until they are needed again.
So, we tend to build the wrong force. We do the same with air power. Sell F-16s to a country and one ensures 20-30 years of relations as they need to keep those suckers flying, and they are cool toys for the sons of the elite to sport around in. Selling them C-130s makes more sense, and some form of low and slow prop-driven ISR/ground attack aircraft.
The other place we get off track with SFA is that too often we are quite frankly making the "Sheriff of Nottingham" much more effective at enforcing the poor governance of some very "Prince John"- like characters out there. This does not endear the US with the populaces of these countries, and it is the populaces that challenge us now, not the states themselves. We need to develop programs better tuned to bringing stability to populaces rather than security to despots.
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