why is not a good idea for countries to try & persue a policy of self -sufficiency?

Answer 1

Simple question of optimal allocation of resources.
Let's take oil production, for example. The US could try to produce enough oil to supply its needs, but that would entail getting oil out of some pretty difficult areas at very high costs. Instead of doing that, the US pays for oil form Iraq, Mexico and Venezuela (which can produce oil cheaper) and invest the money they didn't spend trying to produce oil in more productive areas, like technology.

Answer 2

Self-sufficiency is not necessarily a bad thing, but it does have its drawbacks. Having the ability to be self-sufficient is actually a good thing, but that does not mean that a country should decide to be self-sufficient even if they can be. In times of war, this would be useful, but trading with other countries allows a country to increase its total standard of living, since other countries may be better at producing certain goods and thus have a comparative advantage. Also, consider the fact that many countries that trade with each other are reluctant to go to war with their trading partner, since this would cause the flow of goods to grind to a halt. As such, trade actually can promote peace, and prevent wars. However, since there are many potential trading partners, war still persists because there are enough countries that one does not need to trade with everyone. Self-sufficiency leads to a closed economy which is not open to innovation and thus generally will retard its economic growth.