Which scenario would create a trade war?

Nation A, after exporting certain goods and raw materials to Nation B, begins to import different goods and raw materials from Nation B.

Nation A, after having trade barriers erected against it by Nation B, then puts up trade barriers against Nation B in response.

Nation A, after enjoying a mutually beneficial trade relationship with Nation B, seeks other trade partners to meet its other needs.

Nation A, after increasing its production of a good for export to Nation B, learns that Nation B also produced that good.