The Afghans didn’t defeat the Brits. The Brits eventually imposed their will and put an end their raiding for duration of the British Raj.
We can’t beat them because we’re not ruthless enough. If we applied the methods of the British Raj or, better yet, the Mongols, we could get this over with and go home secure in a real victory.
Of course, there wouldn’t be very many Afghanis left...
Actually not quite true.
The Brits were unwilling to pay the price to conquer and occupy Afghanistan, if they indeed were capable of doing so.
Instead, they established a "buffer zone" in their NW Provinces. These areas are largely occupied by Pathans and other peoples who are "really" Afghans. The Brits didn't try to control this area. They paid off local chiefs to keep the level of violence to a dull background roar. They would make punitive expeditions every few years to keep the inhabitants from getting too out of hand.
It was essentially a policy of "whack a mole" as has been proposed. "Rubble doesn't make trouble" wouldn't work because there wasn't much there but rubble to begin with.
The Brits also fought several real wars with the Afghans themselves, the last in the 1920s.
But the Frontier was never quiet, and raiding never stopped.
I was referring to the Soviets and the Americans - the Russkies are gone, and we’re going soon. The cave-dwellers beat us - granted, with some help from outside both times, but still, they are the ones that had to live with hundreds of thousands of foreign troops, shoot them, bomb them, etc.
The bottom line is that an insurgency in this country would overwhelm the military and police - it would be everywhere, and the cost of imposing martial law in large areas would destroy the government, esp. if the gov’t was the target. That is, after all, why the 2nd Amendment exists - to prevent a tryrannical regime from occupying and ruling this nation, whether that regime originates from outside or inside the nation.