Which event had the immediate effect of causing Germany to declare-war on the Allies? The Berlin Conference of 1885 sets off the "scramble for Africa." Russia mobilizes its forces on Germany's eastern borders. Great Britain declares war on Germany. The Austrian archduke is assassinated.

The event that had the immediate effect of causing Germany to declare war on the Allies was Russia mobilizing its forces on Germany's eastern borders. This action by Russia led Germany to believe that they were under threat from both the east and west, prompting them to declare war on Russia and its allies.