The allies did not try enemy leaders as war criminals after World War I

a) Why do you think they conducted war crimes trials after World War II

b) Do you think they were right to do so

Why?

The Allies conducted war crimes trials after World War II because of horrific acts committed during the war. Nazi Germany had carried out the Holocaust, which was responsible for the imprisonment and slaughter of millions of persons as an act of genocide. People were killed simply because they were Jewish, or a member of other unwanted groups such as the Roma people or homosexuals. Japan had committed similarly horrible acts in carrying out its part of the war. An example would be when Japanese troops took control of the Chinese city of Nanjing in 1937-38. They tortured, raped and killed thousands of Chinese persons in inhuman ways. These were the sorts of actions which were addressed at war crimes trials such as the Nuremberg Trials in 1945-46 and the Nanjing War Crimes Tribunal in 1946.

Right to do so?

Yes, I personally believe that when unthinkable acts are committed during war, those who ordered or committed those acts should be held responsible by the international community.

We should be thoughtful about our own side's acts during war, however. Robert McNamara, who was part of the planning of fire bombings the Allies carried out against Japanese cities, later said, "If we'd lost the war, we'd all have been prosecuted as war criminals." [See the documentary featuring McNamara, The Fog of War.] The Allies were ruthless also in carrying out acts of war, often in ways that annihilated thousands and thousands of civilians - - such as fire bombing of cities or the use of atomic bombs to wipe out cities.