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 for "spreading false information and propaganda", while the UN distanced itself from the content of the exhibition. Greif spent four years examining archives in research of the genocide in NDH, in which he alleged that more than 800 000 Serbians and 40 000 Jews were killed by the Croatian Ustase, in addition to Roma and anti-fascists.