How were war criminals held responsible for crimes against humanity

During World War 1 and World War 2, many war crimes were committed. War crimes are now considered crimes against humanity. Since during the 2 greatest wars the world has encountered, there were no clear rules and punishments that were set. Those crimes were considered necessary to win the war. This prompted the establishment of the International Criminal Court wherein they were tasked to bring war criminals to trial. With the cooperation of nations involved in the War, they set up regulations and punishments. Today, war crimes are punishable by death or imprisonment. Any of which to be granted must undergo trial.