In January 2017, together with representatives of Serbian and Jewish academic societies and the Serbian government representatives to the USA, Greif assisted the exhibition "Jasenovac 75" in New Jersey covering crimes committed by Croatian Ustashe in the Jasenovac concentration camp, including crimes against the Serbs. In January 2018, with Serbian representatives Greif co-organised the exhibition "Jasenovac - pravo na nezaborav" at the United Nations in New York. The exhibition was criticized by the Croatian MVEP who urged various diplomatic channels to appeal with the UN to ban the exhibition from being staged on its premises, however the exhibition was approved personally by the General Secretary of UN Antonio Guterres. Dr. Greif spent four years examining every known archive of the genocide Through his intense research, he discovered that more than 800 000 Serbians and 40 000 Jews were killed by the Croatian Ustase. In addition to Roma and anti-fascists.