Why was it an issue?

The legal basis of the post-Second World War International Military Tribunals

In 1945 there was widespread international support for the prosecution of leading Nazis and some of the German military high command for ordering, condoning or carrying out alleged war crimes and atrocities against civilian populations. However, some specialists in international law were concerned that people might be prosecuted for actions that were not recognised as criminal in international law at the time when they were carried out.  This applied in particular to crimes against humanity, including crimes committed by  governments against their own people.

There was also some concern that Allied political and military leaders had also ordered or condoned actions that could be defined as war crimes or crimes against humanity, such as the fire bombing of Hamburg and Dresden or the dropping of atomic bombs on Hiroshima and Nagasaki.  But, as victors, they were not going to be tried for their actions.

There was also some concern about the trial of soldiers who were obeying the orders of their superiors and would have faced severe punishments for disobedience.  

Then and now the constitutions of most democratic governments prohibit the retrospective prosecution of people for acts which were legal when they were committed, even if those acts were clearly immoral and inhuman. This rejection of retrospective crimes is also enshrined in Article 7 of the Universal Declaration of Human Rights.     

It was important therefore to demonstrate that the atrocities committed during World War II, including the massacre at Monte Sole,  were illegal at the time that they were committed.