Something I don't understand

is why we have to be dependent on other countries for things that we produce ourselves.

Japan's hunger becomes a dire warning for other nations

I'm not suggesting anything like economic isolationism, and I understand the need for trade for things that we can't produce ourselves, but why do we sell cattle to the U.S. for instance, just so we can buy the meat back? There are lots of examples. 'Balance of trade' eludes me...