Why were military leaders and civilians in Japan put on trial after World War II?
Probably to enforce the idea of the existence of personal (and individual) responsibility, even during a war, in relations to crimes against soldiers, civilians and against the humanity.
Before World War II wars were a "free-for-all" in terms of responsibility of nedless violence and destruction; if an army were ravaging, killing, raping, enslaving and pillaging a region after a battle....well, that was bad but...it is war!
Now we have a new idea; if you kill a prisoner soldier, if you torch a village, if you mass-execute a group of people you can expect, after the war, to be called to answer for it!
In particular, if you are a soldier, you cannot use the "I only obeyed superiors' orders" excuse anymore!
The trials of Japanese military leaders after the war were centred upon the "general" idea of them having caused and being responsibke for the planning and suffering of the terrible war unleashed in the Pacific and China.
A lot of military leaders were also on trial for the terrible suffering caused to allied prisoners in captivity (the Bataan Death March, for example) and civilians (the Nanking Massacre, for example).
Civilians were probably extreme right wing politicians and industrialists. The first guilty of orchestrating the manouvers (put in action by the army) to imperialistically expand Japan. The second to take advantage of the resources pillaged from other conquerred countries.
[Former PM Hideki Tojo on trial at the Tokio War Crimes Tribunal]