This paper presents a model where foreign aid bolsters a developing country's proactive counterterrorism efforts against a resident transnational terrorist group. In stage 1 of the game, the donor country allocates resources to terrorism-fighting tied aid, general assistance, and defensive actions at home. The recipient country then decides its proactive campaign against the common terrorist threat in stage 2, while the terrorists direct their attacks against the donor and recipient countries in stage 3. Terrorists' choices in the final stage provide a solid microfoundation for the terrorists' likelihood of success function. In stage 2, greater tied aid raises the recipient country's proactive measures and regime instability, while increased general aid reduces these proactive efforts and regime instability. In stage 1, a donor's homeland security decisions are interdependent with its aid package to a recipient country, hosting resident transnational terrorists. This interdependency and its implications have gone unrecognized to date. Copyright 2011 Oxford University Press 2010 All rights reserved, Oxford University Press.