How did the assassination of the archduke of asturia-hungary lead to war between russia and germany ?

Basically this:

AH asks Germany if they'll defend them, Germany says yes thinking it won't have to deliver on that promise

AH Forces Serbia into war, Russia hops in to defend Serbia

Germany, following through on it's promise, also declares war on Serbia.

Russia assures Germany that it's only mobilizing on AH

France is allies with Russia, but doesn't join the war.

Germany get's paranoid that both France and Russia will invade it since Germany boarders them both, decides to go to war with both of them just in case.
 
Germany and Russia are now at war.

The person who assassinated the archduke of Asturia -Hungary was Serbian and Asturia decided to declare war on Serbia . Serbia was allied with Russia and Asturia was allied with Germany and so Germany attacked into Russia .