We develop and test a theory, based on the Stolper-Samuelson Theorem, of the effectiveness of sanctions. We treat sanctions as exogenously imposed changes in a country's exposure to international markets. In a country with an open-trade regime, owners and intensive users of the abundant factor of production hold economic and political power. In a country closed to trade, however, economic and political power rests with owners and intensive users of scarce factors. Thus, if real rates of return to the abundant factor decline during sanctions against a trade-open country, or real rates of return to the scarce factor decline during sanctions against a trade-closed country, we expect these economically and politically powerful segments of the targeted country to push hard for policy changes that would bring about an end to sanctions. Statistical analysis of sanctions episodes initiated between 1971 and 2000 provides support for the paper's expectations.