Question

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

Answer 1

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.  

Answer 2

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.