"the operative factor in the Holocaust"? Failed to get this

Because Jewry was victims of the Holocaust. The phrase "the operative factor" sounds like the factor of executor to me. What does it mean?

Context:

The Origins of Totalitarianism[edit]
Main article: The Origins of Totalitarianism
Arendt's first major book was titled The Origins of Totalitarianism (1951), which traced the roots of Stalinism and Nazism in both anti-Semitism and imperialism. The book was opposed by the Left on the grounds that it presented the two movements as equally tyrannical. She further contends that Jewry was not the operative factor in the Holocaust, but merely a convenient proxy. Totalitarianism in Germany was, in the end, about terror and consistency, not eradicating Jews.

More:
https://en.wikipedia.org/wiki/Hannah_Arendt#Eichmann_in_Jerusalem:_A_Report_on_the_Banality_of_Evil