Why is it so difficult to disrupt foreign fighter flows? Governments know that foreign fighters can be a threat to both domestic and international security, yet they struggle to prevent would be foreign fighters from leaving their country and track them subsequently. This article seeks to open up the black box of counter-mobilization by examining the ways in which states deal with foreign fighter mobilizations. It argues that there are two main reasons why they may be unable to effectively contain foreign fighter mobilizations. The first is the challenge of inter-state intelligence cooperation due to conflicts of interests between countries. Linked to this are problems in how intelligence is shared. The second reason is the tension, at the domestic level, between collection and prosecution concerns in the investigation of foreign fighter networks. Countering foreign fighting is not the same as countering terrorism, and sometimes the two objectives are in opposition to one another. The article concludes by calling for more research on how states counter foreign fighter mobilizations, particularly focusing on the activities of intelligence, security and police agencies. Counter-mobilization is an important intervening variable that shapes the scale and nature of foreign fighter mobilizations to a greater extent than hitherto reflected in the literature.