From Denis Brian’s interview on Booknotes:

“He was hated by the Nazis because he was almost regarded as the representative of Jews in Germany, the representative of Jewish intellectuals. And he spoke out against the Nazis. And, in fact, when they put a price on his head and people threatened to kill him and he was in Belgium and being protected by the queen of Belgium, who was a friend of his, strangely enough, and who secretly -- she had detectives guard him, when his wife begged him to stop talking out against the Nazis, to keep quiet, his life was in danger, he refused to keep quiet. He said, ‘I wouldn't be Einstein if I kept quiet.’”