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.
The US just left. They left with egg on their face and not looking so superpowery but neither Vietnam or the USSR were in a position to do anything to the US. The US lost, but the US was not vanquished. Actually they basically went home and life just continued on.
The Nuremburg Trials were possible because the Allies were in direct control of the territory of Nazi Germany and had removed it's government. Below someone brought up the various Yugoslav conflicts, but even then, they stopped and then started rounding up the former leaders because NATO basically made it happen. NATO did forceably end those wars.
Wait, what? At the very end US Troops were evacuated from Saigon, and the city was taken over by the Viet Cong. That was the end of the Vietnam war. South Vietnam was reunified with North Vietnam. It's still a communist country.
I'm very sorry, but exactly how is that not complete and total defeat, except under ridiculous exceptions like, "it wasn't really a war because: Congress"? What a disservice to everyone that fought.
The war escalated to the point where LBJ could neither stop, nor continue without basically destroying his legacy (as well as humanitarian and social goals), so he just kept fighting.