AskHistorians

It's much harder to fight than it is to perform. Guerilla warfare generally succeeds by making the fight cost more for the foreign power than it's worth to them.

The polity waging guerilla warfare is usually defending their home, or their vision of it, and has nowhere else to go. As such, they have every incentive to hold out as long as possible. The foreign power is often only there for abstract interests, like economic concessions or 'anti-communism', and thus will not willingly endure the same level of losses as the polity waging guerilla war. They'll cut their losses after a certain amount of pain.

We're just on the other side of the equation in the 20th and 21st century - the foreign intervening power.

US COIN is actually very refined in theory, but requires cooperation and alignment of goals between the civilian US government, the US military, the occupied/liberated (depending on your point of view, or the war) civilian government, and the occupied/liberated military.

... it rarely comes together so smoothly.