Isolationism does not work. It didnt work before WWI or WWII. Membership in the United Nations allows the US to retain its influence in the world and continue peaceful alliances with other nations. It is the UN's responsibility to serve humanity beyond borders and the United States' membership has always been a big reason why there has not been a third world war. There needs to be a place where big and small countries can stand on equal footing and discuss things peacefully.