Following World War II, there was a deliberation that something like that should never happen again - people didn't want to see huge masses of people being killed anymore just because they're from a different race/religion, etc.
With the genocide of jews - this was the common idea and the term genocide originated out of the second World War.
I choose World War II.
It was not just about armies fighting each other but also concerned the
decimation of an entire race of people.
During WWII, the Germans tried to
wipe out the Jews in Europe and almost succeeded had not the allies arrived and
liberated the camps where the Jews were held. This also led the establishment of international laws to prevent genocide and convict those who perpetuate it.
