Christopher Hitchens wrote a whole book on the subject and discussed it at length in multiple interviews, I have included the link to one such video below:

https://www.youtube.com/watch?v=lfcMW1OEw4o

As far as I understand it, the main allegations relate to Vietnam and Cambodia, specifically the allegation that Kissinger deliberately orchestrated the undermining of the original Paris Peace Accords in order to help Nixon win his Presidential Campaign.

He is also accused of planning illegal bombing missions and raids into Cambodia.