The idea that military force cannot defeat the insurgency is rubbish. It's just in this case it's politically pointless, because the cost cannot match the benefit
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I did a bit of thinking about this statement while reading up on the "Night Letters" phenomena being employed by the Taliban, and in this day and age I don't think military force can defeat a very critical component: insurgent will. Their will/motivation is an important target that is very difficult to attack when his IO effort out-cycles yours, you're already considered an invader, and the government you are partnered with is not nearly as transparent as it should be.

For that effort, the full spectrum of resources need to be brought to the table, and frankly, force is probably about 20-25%.