Answer
To stop the Holocaust the United states and other countries could have passed laws that made anti-Semitic illegal.
Step-by-step explanation
Holocaust was genocide which was carried against Jews and other groups. Millions of people including homosexuals, Jehovah witnesses, soviet prisoners of war and political dissidents also suffered mostly oppression and death under Nazi tyranny.
Antisemitism was the discrimination against the Jews. To prevent this before the war the nations could have passed the anti-Semitic law and make it illegal so as to protect the Jews and other ethnic groups from being hated and discriminated