This paper examines the effects of the Operation Frontier Shield pulse operation to defend Puerto Rico from non commercial drug trafficking. The operation clearly forced air and surface smuggling away from Puerto Rico and indirectly lessened the positive testing rates of armed forces inductees there. The concept of pulse operations appears to be successful for several years after the initial pulse. New research is reported about the thresholds for deterrence at different rates of interdiction for lethal apprehension (less than 1%), personal apprehension (2 to 4%) apprehension of associates (4 to 13%), losses of equipment plus drugs (13 to 30%), and drugs only (greater than 30%). These thresholds were derived from interviews with smugglers and verified by analysis of three successful operations. Interdiction rates that are below the thresholds have little impact on deterring drug smugglers and those that are substantially above the thresholds have risk levels so high that the traffickers abandon smuggling. A preliminary model showing the effect of interdiction on smuggling deterrence is documented. The research indicates that it is important for commanders to understand the minimum thresholds for interdiction deterrence so that proper employment of scarce interdiction forces are made.