They were dominating the world and they needed help to do more and they saw that they were different and used the people as slaves
The countries of Europe were very divided after World War I and were in no mood to establish a world changing organization dedicated to promoting world peace and understanding. The U.S not being a european country( an outsider) may have helped fix problems as a median. But, congress was not interested in european affairs.
Options??what are the options
The 2 nations that emerged as superpowers after WW2 were the Soviet Union and the U.S.