United we stand, divided we fall. This shows that a tree cannot a forest because it not going to survive the hash weather. Countries need each other to trade and survive in the long run because each country natural resources and area of specialization differs from the other.
This is why you see that we have UN and BRICS because you need to be part of a group with the same vision to help each other grow together, defend and fight for each other when being attacked by the enemies.
Yes, we need all of us together, otherwise it is not going to be okay. One nation can't be self sufficient in everything, sure some of the things could be true, but not in EVERY thing, so you need other countries to trade and make sure that you have access to some other stuff.
This is why we need to make sure that we are dealing with peace, and not war. Whenever there are wars around the world, that means terrible for the economy of every nation, even in the nations that are not in the war.