Incorporating pollution emissions from international transportation into a model of strategic trade and environmental policies, we investigate the effect of trade liberalization and environmental regulation on national welfare and the environment. Our model includes imperfectly competitive markets for international transportation and final products. We find that trade liberalization may reduce each country's welfare unless some level of environmental regulation on international transportation is in place. When international trade is liberalized initially, a mutual increase in the common emission tax rates may improve each country's welfare. However, when international trade is highly protected initially, imposing an emission tax may reduce welfare.