To find the assumption, we need to understand the argument made in the paragraph. The argument made is that it would not be appropriate to issue a blanket pardon for the executions that happened during the war. This is because the individual cases cannot be review fairly now since the norms have changed over the years. Therefore, a legal pardon cannot be issued.
This means that the passage is assuming that there would be individual differences between the soldiers who were executed and that some deserved deaths more than others. It does not take into consideration that those who were executed would deserve or not deserve death sentences equally.