what treaty forced Germany to declare war on Russia on August 1, 1914?

Answer 1

Treaty of Alliance Between Germany and Turkey

http://www.yale.edu/lawweb/avalon/turkgerm.htm
http://www.lib.byu.edu/~rdh/wwi/1914/germturk.html

Answer 2

a treaty known as the Dual Alliance, signed on October 7, 1879 was a fundamental factor that contributed to Germany's declaring war on Russian in August 1914. This treaty stipulated that if either Austria-Hungary or Germany was attacked by Russia, that the other would pledge their aid, or (in the case that they were attacked by another European Power, i.e. France) benevalent neutrality. The Alliance treaty was created primarily by German Otto von Bismark.