The US hasn't "done the above" because they tried and failed. The Afghan Army were supposed to be the US's local stooges.
Why the US failed was because US troops were true believers in massacring innocent Afghan civilians. Their relatives sought revenge by joining the Afghan Army then turning their guns on their "team mates" - US troops - even if it meant their own deaths.
You will find that cynical Machiavellian policies of the sort you mentioned above depend to a great degree on deterrence, which doesn't work when your enemy no longer has anything to live for and decides to go kamikaze.
Why the US failed was because US troops were true believers in massacring innocent Afghan civilians. Their relatives sought revenge by joining the Afghan Army then turning their guns on their "team mates" - US troops - even if it meant their own deaths.
You will find that cynical Machiavellian policies of the sort you mentioned above depend to a great degree on deterrence, which doesn't work when your enemy no longer has anything to live for and decides to go kamikaze.