Russia
Russia came to the aid of Serbia because they were slavs and the Czar felt a special kinship to Serbia and not Austria-Hungary. Austria-Hungary were allied with the Ottoman Empire, Germany and Bulgaria. Serbia wanted its independence from the Hapsburgs and knew that the Romanovs would support them militarily.
Yes, Russia supported Serbia during World War 1.
Germany
Russia chose to support Serbia during World War I because of their shared Slavic heritage and historical ties. Russia saw Serbia as a fellow Slavic nation and felt a sense of duty to protect and support them against the aggression of Austria-Hungary. Additionally, Russia saw an opportunity to expand its influence in the Balkans by supporting Serbia.
Because of his death Austria-Serbia declared war on Serbia, Russia came to defense and soon the World War one began in Europe.
they promised to support Austria-Hungary after it threatened to invade Serbia
Germany guaranteed Austria-Hungary its support in the conflict with Serbia.
they promised to support Austria-Hungary after it threatened to invade Serbia
Serbia, The Triple Entente - France & Britain.
Germany promised support to Austria.
Astro-Hungary declared war on Serbia. In turn Russia came to Serbia's aid with a few other countries.