1) After WWI, the US became increasingly isolationist as the public did not want to be involved in overseas conflicts and feared the costs of future wars. 2) Key policies like rejecting the Treaty of Versailles, passing the Fordney-McCumber Tariff, and reducing immigration levels through quotas and literacy tests aimed to isolate the US economically and politically from other countries. 3) This isolationism ultimately hindered global cooperation and damaged world peace as the US refused to join the League of Nations.