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 Serbia
Serbia was in WWI because Austria-Hungary declared war on them.
No, Serbia was not part of Russia during World War 1. Serbia was an independent country that was allied with Russia during the war.
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).
There were many reasons and here are some.First House of Karadjordjevic (Ruling royal house of Serbia and later Yugoslavia) was tied with Russia, this didn't suit AH.Second reasons was that Serbia wanted to be economically independent of AH and AH wanted Serbia as her satelite state. Which eventually led to so called Pig War or Customs war in 1906.Third reason was that in southern parts (today Slavonia and Vojvodina) of AH there were thousands of Serbs who wanted to unite with their motherland.Fourth was Annexation of Bosnia by AH in 1908 where majority of population was Serbs. There were alot of protest in Serbian cities who wanted war with AH.Fifth was AH intervention and making of state of Albania. Kingdoms of Serbia, Monthnegro and Greece liberated all of today parts of Albania and AH didn't want Serbia to connect with sea.