Make Operation Vegetarian go into effect. Now, I don't know what would need to happen for this, but that's your job. Also, I wonder how this would affect the Soviet Union in relation to it?
Have the US stay neutral. With the Germans and Soviets stalemated in the Ukraine, the blockade of Britain worsening and intelligence revealing the Nazi have little knowledge of what Anthrax even is, Churchill chooses to end the war.