Final answer:
World War I (a)
Step-by-step explanation:
The historical event that seemed to bring Bernays' career to an end was World War I (a).
During World War I, there was a rise in anti-German sentiment in the United States due to Germany's actions leading up to the war. Additionally, the Bolshevik revolution in Russia intensified concerns about communist revolutionaries overthrowing capitalist governments in western Europe and North America. These events contributed to a climate of fear and censorship, which ultimately impacted Bernays' career.