When Russia found out that Austria-Hungary was declaring war on Serbia, they came into alliance with Serbia for several reasons.
There was no way Serbia could avoid Yugoslav war, because if there was Serbia would have done it...
Serbia was in WWI because Austria-Hungary declared war on them.
Serbia Serbia
The 1st country to declare war on Serbia was Austria-Hungary.
The war between Serbia and Kosovo was going on. The war started because Serbia wanted more territory from Kosovo.
Austria-hungary attacked Serbia
Russia
A event that sparked the war was the assassination of Archduke of Austria-Hungary in 1914. Gavrilo Princip, a Serbian nationalist was responsible for the assassination. He believed Serbia should be independent from Austrian rule. Austria-Hungary later called war on Serbia which resulted in both Serbia and Austira-Hungary's alliances to join the war. Countries continued to join. The United States later entered the war for a different reasons such as the sinking of the Lusitania, the Zimmerman note etc.
At the begining of the war Serbia was allied with Montenegro, Russia and Greece. Greece stayed neutral in war, while Montenegro and Russia helped Serbia to fight with Austria-Hungary. When Serbo-Austrian war became World war Serbia alongside Russia and MOntenegro was part of Entante (Allies).
Serbia
no