FWIW if you want a quick summary, the most common claims that Kissinger was a war criminal and my rebuttal of them:
Bombing of Cambodia - This was illegal under U.S. law, and would be considered a breach of international law in terms of belligerency. It was a violation of the UN Charter, and “illegal.” However confusing as this may be, it doesn’t make the perpetrators of it a war criminal, war criminal has a more limited and specific definition by international law and treaties. This actually isn’t terribly dissimilar to the 2003 Iraq war–again, an illegal action under the UN Charter and not a “legal war”, but not a war crime.
What about civilian deaths in Cambodia from the illegal bombing campaign? Evaluating those deaths actually requires looking at the motivation and intent of the belligerent. In Cambodia all evidence we have is the U.S. bombing campaign was targeting covert North Vietnamese bases in Cambodia. Such a bombing campaign will all but certainly kill civilians. It is not a war crime to kill civilians. It is a war crime to target civilians. There is no evidence the U.S. bombing campaign in Cambodia was targeting civilians.
The East Timor Genocide - East Timor or Timor-Leste is a small island country in the Indonesian archipelago. It was a Portuguese colony which is how it ended up not being part of Indonesia proper. In the mid-1970s the ruler of Indonesia–a U.S. ally, Suharto, wanted to invade East Timor because he didn’t want them to be independent and he claimed that their government had communist leanings. Kissinger supposedly approved this invasion and Suharto did ask us first. In the ensuing occupation a recognized genocide occurred. During the time of that genocide and the 25 year illegal occupation, we continued to support Indonesia and sold weapons to them.
This is not a war crime because we were not at war in East Timor, were not engaged in belligerent activities, and were not a party to conflict. Selling weapons to a belligerent who commits war crimes, does not create “war crimes by proxy.” Countries like Switzerland and Sweden that continued to do trade with the Nazis during WWII did not incur war criminal status for their leaders by doing so.
Chilean Allende Coup - This one is one that was likely illegal under U.S. law and definitely violates the U.N. Charter, but we were not direct belligerents at all, which makes it hard to prosecute as a war crime. We provided covert support to Pinochet, but we did not directly commit the atrocities–Pinochet did. The coup was illegal but not a war crime. Pinochet was a war criminal but we don’t become war criminals by transitive property through our connection to him.
Now another layer on top of all of this–Kissinger’s scope of authority and role in all of this is somewhat questionable. It’s pretty well understood he was deeply involved in East Timor, it’s less clear just how culpable he is the other two. He was involved in advising Nixon, certainly. But the Secretary of State doesn’t order the CIA to run coups, the Secretary of State doesn’t order our military to start bombing Cambodia. Richard Nixon did those things–none of which I believe are war crimes for the reasons I stated, but if they were, it would almost certainly be Nixon that bore culpability. The role of non-command civilian political advisers in conducting war crimes isn’t one of total immunity–there is some precedence at least under the Nuremberg trials for prosecuting civilian leadership, but those cases were much more direct than anything alleged of Kissinger.
For example Albert Speer was Minister of Armaments was actually overseeing the use of slave labor directly. Some of the other Nuremberg non-military defendants, may not actually have been prosecutable under subsequent war crimes regimes setup after WWII. For example Julius Streicher was a propagandist who got convicted of Crimes Against Humanity, and executed for it, but he never directly caused the death of a single person. He just operated virulent anti-Jewish propaganda outlets.
While this was considered legitimate under the structure of the Nuremberg tribunals, I’m not sure he, or Ribbentrop, could easily be convicted under the modern regime around war crimes.