A question from Yahoo! Answers:
Should the U.S. be involved in International Trade at all? Why cannot we produce everything by ourselves?
First, there are minerals that the U.S. does not have or has in insufficient quantities; sometimes, the minerals are there, but mining them domestically is more expensive than mining them internationally. Second, there are agricultural commodities that simply do not grow in the U.S. (bananas and coffee are prime examples).
But more importantly, why should the U.S. produce everything domestically? As Adam Smith put it in The Wealth Of Nations,
It is the maxim of every prudent master of a family, never to attempt to make at home what it will cost him more to make than to buy…What is prudence in the conduct of every private family, can scarce be folly in that of a great kingdom. If a foreign country can supply us with a commodity cheaper than we ourselves can make it, better buy it of them with some part of the produce of our own industry, employed in a way in which we have some advantage.