In Why the U.S. Loses ‘Small Wars’, Larry Kahaner (AK-47: The Weapon that Changed the Face of War) goes back to Small Wars, a book written in 1896 by C.E. Callwell, a colonel in the British army, who learned its lessons in the Second Afghan and Boer Wars:
He notes that the primary object in a small war is to force insurgents to fight on the regular force’s terms by drawing them into conflicts in which their superior firepower and discipline could prevail. Unfortunately, the history of small wars has shown that insurgents play hit and run — striking boldy and then retreating quickly, and rarely engaging the larger force head on.The other, and much bigger obstacle to winning small wars, brings a moral dilemma. According to Callwell, to win small wars, mere victory isn’t enough, the enemy must be thoroughly and utterly destroyed to the last man, woman, and child — which means enormous civilian casualties. For citizens of most modern democracies, this is an unacceptable stance. The level of violence and barbarism it would take to beat an insurgent force — torture, wholesale executions, leveling of towns — is a place where most democracies refuse to go. This keeps victory out of reach.
Small wars are also lost because of the larger army’s lack of national commitment which ends in inadequate or misspent funds and deployment of too few troops. For insurgents fighting for their own soil, the commitment is 100 percent. If they lose the war they lose everything. Without ‘skin in the game’ national commitment by the larger force’s country usually wanes.