Brad DeLong cuts through the all the non-sense about fear of trade and explains that worst thing that can happen with open trade is no trade at all:
One--accurate--path through the swamp is to distinguish between (i) productivity improvements abroad that make foreigners more efficient at producing what we import and (ii) productivity improvements abroad that make foreigners more efficient at producting what we export.
The first set of productivity improvements is a boon for us: the prices of goods we import fall as foreigners become more efficient at producing them, and our standards of living rise. The second set of productivity improvements is a bane for us: as our exports face more competition, the prices we can charge for our exports fall, and so our exports buy less in the way of imports, and our standards of living fall. How bad can this second force be? Well, in the limit--in which foreigners become so good at making stuff that there's nothing we make they want to buy at a price at which we are willing to sell--we are as badly off as if there were no international trade at all. The worst thing that engagement with the international economy can yield is the same as... the no-trade autarky outcome. What's at stake isn't our absolute impoverishment: it's the loss of some (or most?) of what had been our gains from international trade.