What would happen if the whole world just stopped trading with each other? Would the countries find a way to get the resources that they need or would they just do without?
Broadly, two immediate things come to mind. Some things, we trade for because we legitimately do not have the resources that are only available in other parts of the world, so we would just need to go without. Second, and also as important as the first, is that some things we buy from overseas because we have a huge opportunity cost when we make them here at home. If no nation were to trade, everyone in the world would be living in poverty - if at all.