Assassination of Archduke Franz Ferdninand at Sarajevo
Killed by Serbians, deteriorating relations between Austria and Serbia

 
Nationalism
Serbia- Pan-Slav Movement
Alsace-Lorraine stirred up French nationalism

 
System of alliances
Triple Entente; Triple Alliance
Each power supported its allies, which dragged all the powers into the war

 
Bosnian crisis
Annexation of Bosnia-Herzegovina angered Serbia
Russia was humiliated, fueled anger against Germany

 
Mobilisation
Mobilisation of Austria-Hungary started the mobilisation of each other power, leading them into war

 
Balkan Wars
Serbia became a more powerful nation, a threat to Austria-Hungary, which strained their relationship.
Failed to balance power of Balkan League

 
Arms race
The increasing number of deadlier weapons meant that powers felt threatened by each other

 
Militarism
Militarism was only a belief, and the preparation for war didn't lead to war itself

 
Colonial and trade rivalry
Imperialism caused tensions between countries, but ultimately the disputes did not have a large impact on the war