A three-stage game investigates how counterterrorism measures are affected by volunteers’ choice in joining a terrorist group. In stage 1, the government chooses both proactive and defensive countermeasures, while looking ahead to the anticipated size and actions of terrorist groups. After radicalized individuals choose whether to join a terrorist group in stage 2, group members then allocate their time between work and terrorist operations. Based on wages and government counterterrorism, the game characterizes the extensive margin determining group size and the intensive margin indicating the group’s level of attacks. Comparative statics show how changes in wages or radicalization impact the optimal mix between defensive and proactive countermeasures. Higher (lower) wages favor a larger (smaller) mix of proactive measures over defensive actions. In the absence of backlash, enhanced radicalization of terrorist members calls for a greater reliance on defensive actions. The influence of backlash on counterterrorism is also examined.