Secrecy and informal organisation produce, sustain, and reinforce feelings of loyalty within intelligence and security services. This article demonstrates that loyalty is needed for cooperation between intelligence partners as well as within and between services. Under many circumstances, loyalty plays a larger role in the level of internal and external collaboration than formal work processes along hierarchical lines. These findings are empirically based on the case study of Anglo‒Dutch intelligence cooperation during World War II. By demonstrating that ‘loyalty’ critically affects the work of intelligence communities, this article contributes to current and future research that integrates history, intelligence studies, and research on emotions.