What is an anti-jewish policy or action that should have warned the world that the nazis needed to be stopped immediately.

The Nazi party, and Hitler himself, were obsessed with creating the "perfect world" this included having a superior and inferior complex towards certain sects in society. The German political climate at the time of Hitlers rise to power was one of sheer desperation. The great depression was at its worst, and when people are desperate they will turn to the person who provides not only a solution, but an explanation for all the bad that is happening around them. Hitlers scapegoat was the Jews. He blamed them personally, and so when he rose to power in 1931-33 he spread propaganda about them. The German people, being desperate for answers and solutions, listened. Even if the German people did not agree with Hitler, they could not voice their opinions in defiance of this without being beaten or worse. 

This is where the line should have been drawn. Hitler had such absolute control, that people could not voice their own opinions. This was not necessarily a policy or action, but rather a general atmosphere. The German people were either desperate enough to believe what Hitler and his propaganda told them, or were to afraid to speak out.

I hope this helps :)