Officials from all factions in (for one example) the Bosnian civil war have been tried by the ICTY. So unless all sides suffered "utter defeat", that is demonstrably untrue.

One could argue that prior to the establishment of modern tribunals starting with the ICTY (but also including the ICTR and ICC), that was the case. And there might be other reasons that the US, in particular, is difficult to hold accountable. But its pretty clear that it is no longer the case that "to be charged with war crimes requires utter defeat", or even mere defeat.