International -- and, for that matter, any kind of trade -- makes people better off than being self-sufficient. Unequal endowments, whether they're in the forms of natural resources, labor or capital, make for comparative advantages in production.
For example, Alaskan citizens can produce oranges just as Floridians can produce king crab legs. It's simple. Alaskans could build greenhouses that simulate Florida's weather conditions and Floridians could build aquariums that simulate Alaska's water conditions. Both would be self-sufficient in both products, but Alaskans would pay through the nose for oranges and Floridians for king crab.
Probably most of us would agree that preventing trade between Floridians and Alaskans would be stupid and costly. That conclusion would change not one iota if Alaska happened to be another country, instead of another state.
Preventing international trade does benefit some people. In my Mexico/U.S. example, Mexican computer chip manufacturers and U.S. tomato producers would benefit from outlawing trade. It would enable Mexican computer chip manufacturers and U.S. tomato producers to charge their customers higher prices, thereby making for higher profits and wages.
Trade barriers are an excellent means to higher wealth for the few but lower wealth for the many.