Which event had the immediate effect of causing Germany to declare war on the Allies?

It was the German invasion of Poland in 1939 that led Britain and France to immediately declare war on Germany, which immediately led Germany to declare war on Britain and France.

Answer;

The event had the immediate effect of causing Germany to declare war on the Allies was, Russia mobilizing its forces on Germany's eastern borders.

Explanation;

In July 31,1914 Reacting to the Austrian attack on Serbia, Russia begins full mobilization of its troops. Germany demands them to stop. In August 1 the same year, Germany declared war on Russia, France and Belgium and begins full mobilization of its troops.