The refusal to trade with aggressive countries was called

I think it's the League of Nations, which was formed during the first world war, for stopping the trade with aggressive countries, and to replace secret deals and war. The formation of the League of Nations concluded the first world war. It was meant for the settlement of the international disputes.

Use the formula: (n-2)×180, where n is the number of sides(18-2)×180= 28800, now this is the full exterior angle of 18-gon so you divide it by 18.28800÷18=160