Insurgents neutralize collaborators to tie down a stronger army defensively [causal]
In the early days of the Algerian War, once the FLN realised it was not strong enough to take on the powerful French Army, it concentrated its attacks on the native police loyal to France. Result: a deadly loss of morale among the police, with defections to the FLN, and the French Army defensively reduced to protecting the police, instead of concentrating on active "search-and-destroy" missions. The "insurgents" in Iraq have learned from this strategy with deadly effect.
XREF: This connects to the FLN's Algeria strategy and its direct echo in Iraq — a classic asymmetric warfare tactic of bypassing the strong power and targeting its local allies.
Horne, Alistair, A Savage War of Peace- Alge…, loc. 49