Well that's the problem. Free trade can only happen if everyone does it. And even then, problems can occur, but it may tend to balance out. Since every country would have plus trade with some countries and negative with others.

Anyway, lets consider only two countries for a moment.

If you go the free trade route and buy all my stuff while I block my people from buying your stuff, it's a problem.

You give me gold bars every month while I give you silver, eventually you run out of money.

And let's add on that my geopolitical goals are counter to yours. Now you are funding my ambitions which directly harm you.

Free trade sounds like the correct answer, but what do you do when your rivals choose to take advantage?