There is this odd thing about the human brain that as soon as it is in a group and have tools of violence at hand it will often start planing to take over the society
ISIS was a good example, they were being quickly defeated by pretty much everyone involved in the Syrian conflict.
Individuals might not have heavy weapons to start. But with enough time, weapon destructivness reaches a parity.
Now think about whether either of those conditions apply in a particular scenario, then remember you need both. Afghans obviously hated being invaded and helped "their guys", they also had Peshawar.
The captured vehicles could often be serviced by either side. Also, some vehicles could be traded with foreign partners'. I bet you there are multiple T-90 and even more interesting vehicles in Yuma Proving Ground and other compounds, disassembled to the last screw.