What instead he based his assumptions were was

[suprunp]

What instead he based his assumptions were was the idea that because he believed that America was ruled by Blacks and Jews that therefore they would not be able to fight successfully in the Second World War.
(Why Hitler Lost the War: German Strategic Mistakes in WWII; YouTube)

Would you be so kind as to tell me whether I've heard it correctly?

Thanks.

 

[GoesStation]

You have, and given how carefully the lecturer is speaking, it's a surprising error. In the very next sentence he makes a very clumsy circumlocution to avoid what he must have been taught was the mortal sin of splitting an infinitive.

He meant on,​ not "were".