^ these answers should be pertinent.
Simply put, the Russians were involved because the Germans declared war on them. It really wasn't a matter of "having/not having" to join the war. Now, the Russians certainly did not half to begin mobilizing their forces in late July, but it's hard to see how else they could have deterred the Austro-Hungarians from attacking Serbia. They could also have simply stayed out of the whole affair, but after foreign policy set backs in 1905, 1908 and 1912-13, that to was unlikely.