no country in the world can live independently of other countries

No country in the world can live independently of other countries. Why cant a country live independently of other countries? For example in one country there could be a lot of wood, but no oil. And the other has oil, but has no wood. So these countries can trade and the first country will have oil, the second one will have wood. This trade is very favorable and we all know that both countries will die without each others help. Or for example some poor countries cant live without help from rich countries. Like some African countries need help from European countries.