How/why did the world allow an event like the Holocaust to happen?

Following the Enlightenment (late seventeenth century – early nineteenth century), there was a growth in nationalism. The rise in nationalism intensified the rise in antisemitism, which had also been growing since the Enlightenment. The First World War (1914-1918) strengthened these feelings of nationalism across Europe, as nations were pitted against each other.

although the legal justifications for the trials and their procedural innovations were controversial at the time, the nuremberg trials are now regarded as a milestone toward the establishment of a permanent international court, and an important precedent for dealing with later instances of genocide and other crimes

c) legal battles last five weeks after the election due to the narrow victory in several states.