why was their tension between austia, russia and serbia in world war one?

why was their tension between austia, russia and serbia in world war one?
please write it in full words, need it for my history essay and i really dont have a clue, please help:(

In the early years of the 20th Century the Balkan region where Serbia is located, had experienced great changes. Turkey had controlled a large part of the area under the Ottoman Empire and there had been two wars fought prior to World War 1 to drive Turkey out. Serbia had emerged as a new country but ethnic and linguistic Serb people still lived in other areas of the Balkans that Serbia thought should be under their control.

Meanwhile, the Austria Hungary Empire had grabbed a bit of territory itself and controlled what are today Slovenia, Croatia and Bosnia Herzegovina. Serbia felt that the Serbs in these areas should be with them not Austria.

Russia always looked upon the Slavic people throughout Eastern Europe as ethnic “kinsmen” with a similar written language and saw themselves as a protector.

To make matters more complicated…..much of the Northwestern Balkan area (Slovenia and Croatia in particular) had been historically influenced by the Roman Empire and used a Roman-style alphabet and adopted Roman Catholicism. The Eastern and South Eastern parts of the Balkans (Particularly Serbia Macedonia and Bulgaria) had been influenced more by Russia and had adopted a Cyrillic alphabet and Orthodox Christianity. And other parts like Bosnia and Albania had remained Islamic under Turkish influence!!!! These are a large part of the divisions that still plague the region.