How was World War II related to religion?

World War II was related to religion insofar as one of the main protagonists, Nazi Germany, was hell-bent on wiping out the entire Jewish population of Europe. This is not say that the Nazis' genocide was motivated by religion; their warped ideology was steeped in cultural and biological racism. Furthermore, the Nazis regarded Judaism as not just a religion, but a race.
Even so, Judaism was and is a religion, and it suffered appallingly during World War II. It's been estimated that the pre-war Jewish population of Europe was somewhere in the region of 9 million. Yet by the end of the war, that number had declined significantly due to the Holocaust, in which over 6 million Jews were murdered by the Nazis.