The US and Japan rely heavily on other countries for their economy, as does every single economy on Earth. Say you put a wall around a flourishing city; nothing in, nothing out. Everyone will die. It's that simple.
Expand this any way you like. It is basic physics. An organism can only maintain itself via material from outside itself, whether that is sunlight and soil or imported consumer goods. Even the ground we walk on is replenished by volcanic activity. And an organism must expel waste, whether this waste is gas or a glut of government-subsidised soy beans or even the trillions of tons of garbage that "First World" countries have polluted their own and poorer countries with.
The cold hard fact is, no country can succeed unless
1) it is small or lucky enough to have a large amount of unused land/resources which are not being used by the population.
2)it has something that people in other countries want.
3) other countries are willing to supply that country with (at the very least) raw materials
Even in the early days of e.g. Europeans' colonisation of North America, the home countries were fundamental for currency and goods. Millions of animals in Canada were slaughtered for money from outside. The Gold Rush would have meant nothing if there was no-one to give it to in exchange for products. Human culture is now so large and so voracious that we will eventually (and much sooner than we think) get to the stage where we are basically living in one, sealed town called The Earth.
Life managed to survive for billions of years because it never ran out of resources. Humans are not that efficient.