International environmental cooperation is difficult because states disagree on burden sharing and have incentives to free ride. However, interested countries can promote future cooperation through unilateral action that induces technological change in and, thereby, shapes the preferences of foreign countries. How can the effectiveness of such unilateral action be improved? This article offers a game-theoretic analysis of the value of combining unilateral action with trade sanctions, or policies that force foreign exporters to comply with domestic environmental regulations. Trade sanctions can significantly improve the effectiveness of unilateral action, but only when (1) they induce clean technology adoption by exporters in targeted countries and (2) this reduces the cost of clean technology elsewhere in the economy through intersectoral technology spillovers.