In civil wars across the world, certain Islamist groups have competed exceptionally well against their rivals. The conventional wisdom points to either religion or ethnic politics to explain Islamist success. These ideological and identity-based explanations, however, tend to overlook the powerful economic influence that the local business class has over civil war outcomes. Civil war can be modeled as a market for security, wherein protection must be purchased from multiple substate rackets. Using this market model, a close investigation of the Somali case reveals why and under what conditions the interests of the profit-driven business class align with those of ideologically motivated Islamist groups. Security costs are of critical importance to businesses in a civil war, and Islamists are uniquely competitive in lowering these costs. The business-Islamist alliance is therefore driven by rational, economic considerations, which can contribute to the rise of Islamist power.