During the past half century, multilateral trade liberalization has reduced tariffs to historically low levels. The Received Theory of multilateral trade agreements, based solely on terms-of-trade externalities between national governments, offers an explanation that has become the conventional wisdom among international trade theorists. But this explanation displays two puzzles that render it inconsistent with actual trade policy and actual trade agreements. This paper introduces intergovernmental political externalities into a model with terms-of-trade externalities. It delivers results consistent with what we actually observe, and thus resolves the puzzles, if and only if political externalities dominate terms-of-trade externalities.