That's sort of an unhelpful truism. By definition, the losing parties don't have control of a judicial system within which a trial of the winning parties could take place.

If what you mean is "winning parties don't prosecute their own side", then I would also disagree with that, since some countries do believe that the rules they are ostensibly fighting for do apply to themselves:

https://www.theguardian.com/news/datablog/2012/mar/29/afghan...

I wouldn't exactly call US the winner of the Vietnam or Iraq wars

would you consider Vietnam to be in a position to write and have their own side of it become history?

And only once the "middle management" died of old age a decade ago.