How did Russia enter the war?

Although Russia wasn't supposed to be in the war, she nevertheless became involved and eventually played a major role as one of the four big imperial powers. The war was originally supposed to be limited to two countries: Austria-Hungary and Serbia. How did Russia become involved?
The simple truth is that Russia was obligated to join the war because of various alliances and other clauses from other alliances. In 1873, Russia was tied to Germany and Austria-Hungary because of signing into the Three Emperors League. However, Russia pulled out of that alliance five years later. But Russia's withdrawal created a new Dual Alliance with Bismarck and Austria-Hungary. This Dual Alliance promised those two allies aid in the event of an attack by Russia, or in the event of an attack made by a country in which Russia was allies with. This alliance was upheld until the war finally broke out.
Because Austria-Hungary was being attacked by Serbia, and Serbia was being aided by Russia, Austria-Hungary called on Germany for protection against Russia.