If the idea of gains from trade is the most important theoretical concept in international economics, the seemingly eternal battle between free trade and protection is its most important policy theme. Since the emergence of modern nation-states in the sixteenth century, governments have worried about the effect of international competition on the prosperity of domestic industries from foreign competition by placing limits on imports or to help them in world competition by subsidizing exports. The single most consistent mission of international economics has been to analyze the effects of these so-called protectionist policies – and usually, though not always, to criticize protectionism and show the advantages of freer international trade.