Previous- Soviets Crush Weimar in Germany
On August 15th, 1940, Britain declares war on the USSR, swiftly followed by the US and France. They quickly send troops through France into Germany, re-inforcing the Germans and boosting morale.
However, at the same time, the Germans have been forced out of Hanover and forced to move to Cologne. If they lose Cologne, they will be forced out of Germany and probably will not be able to retake it. The US and UK generals have two choices- send the troops through Alsace-Lorraine, fighting through stiff Soviet resistance in Frankfurt, then save Cologne (assuming it holds out that long), launch an assault through Belgium and Luxembourg, or ignore the Weimar government and try to stop the Soviets from advancing further.