What I believe -
Although Austria-Hungary's aim in 1914 was to fight a limited war to punish Serbia after the assassination of the heir to the Habsburg crown, Franz Ferdinand, the crisis quickly flared out of control as some Countries mobilized their mass armies in accordance with their treaty commitments. Although poorly prepared for conflict and lacking essential weapons and unit cohesiveness, the Austro-Hungarians were immediately faced with a two-front war against Serbia and Russia.
Serbia itself didn't want to assassinate Franz Ferdinand, a terrorist group called the Black Hand wanted to kill him because they did not like the fact the Austria-Hungary ruled Serbia and did not like the changes that Ferdinand planned to make.