Tension began to grow due to the struggle for control over the Balkan Peninsula.
Explanation:
Order in the Balkan Peninsula was subject to the collaboration between Russia and Austria-Hungary. Austria-Hungary's power was unstable and under risk, which generated concern, so, in 1908 it added Bosnia and Herzogovina to its territory. This outraged Serbia (which was an independent Balkan country) and also Slavic Russia.
Serbia eventually grew its territory considerably during Balkan wars, which endangered sovereignty of Austria-Hungary. Russia then allied to France and Great Britain (which held tensions with Germany for territory and control over seas) forming the Triple Entente. On the other side, Germany an Austria-Hungary formed an alliance. Soon any conflict would lead into WW1, which was the case of Franz Ferdinand's assasination.
All of the European powers wanted to be in control and all had secret alliances with others. This was all already a problem before the assassination of the duke. The assassination was what finally set all these seemingly small issues into 1.
