Under what conditions do states respond when other states support rebels against them? I develop a theoretical framework that explains when and how states retaliate against foreign rebel support. I propose that retaliation may take two primary forms: retaliatory use of force or retaliatory rebel support. Accounting for non-military responses such as inaction, condemnation, and diplomatic and economic sanctions; I argue that if states retaliate, they may either engage in covert retaliation through rebel support or overtly retaliate through the use of force. More often, states prefer retaliatory rebel support, although this choice may depend on factors such as conventional military capabilities, ethnic kinship within the adversary state, and the presence of an existing rebel group to support there. States often choose rebel support as a retaliatory strategy because it provides an effective response without directly confronting the sponsoring state. By supporting rebels, the responding state signals its willingness to retaliate if the initial support for the rebel group continues, without resorting to direct confrontation. This approach often succeeds in conveying the message, prompting the sponsoring state to withdraw support for the rebels and avoiding direct escalation. This research is one of the first to investigate retaliation against rebel support and its role as a cause of interstate conflict.